Operation Inherent Resolve

Operation Inherent Resolve (OIR) is the U.S.-led multinational military intervention against the Islamic State of Iraq and Syria (ISIS), initiated in June 2014 with airstrikes in Iraq to counter the group's rapid territorial gains, including the capture of Mosul, and expanded to Syria shortly thereafter.^[1] Formalized as Combined Joint Task Force – Operation Inherent Resolve on October 17, 2014, the operation encompassed over 80 coalition nations providing airpower, special operations, training, and advisory support to Iraqi Security Forces, Peshmerga, and Syrian Democratic Forces to degrade ISIS's capabilities and reclaim territory.^[2] Its primary objective was the lasting military defeat of ISIS, achieved territorially with the liberation of the group's self-proclaimed caliphate by March 2019, though ISIS remnants persist as an insurgency requiring ongoing counterterrorism efforts.^[2]^[3] The campaign involved tens of thousands of coalition strikes, the training of over 150,000 local partner forces, and the provision of billions in equipment and logistics support, significantly contributing to the collapse of ISIS's governance structures and reduction of its fighting force from peak estimates of 30,000-40,000 combatants.^[4]^[5] Notable achievements include the recapture of key cities like Ramadi, Fallujah, and Raqqa, bolstered by precision airstrikes that minimized coalition ground troop exposure while enabling ground advances by allies.^[6] However, operations drew scrutiny over civilian casualties, with coalition reports confirming at least 1,437 unintentional deaths from 2014 onward following rigorous investigations, contrasted by independent estimates from monitoring groups ranging up to 13,000, highlighting discrepancies in verification methodologies and data sourcing.^[7]^[8] By 2025, OIR transitioned in Iraq to bilateral U.S.-Iraqi security partnerships, ending the formal coalition mission there in September amid stabilized conditions but persistent ISIS threats, while continuing advisory roles in Syria against resurgence risks.^[9]^[10]

Background and Rationale

Rise of ISIS and Precipitating Events

The Islamic State of Iraq and the Levant (ISIL), commonly referred to as ISIS, emerged from the remnants of Al-Qaeda in Iraq, a jihadist group founded in 2004 amid the U.S.-led invasion of Iraq. Severely weakened by U.S. counterinsurgency operations during the 2007-2008 surge and the death of its leader Abu Musab al-Zarqawi in 2006, the group reorganized under Abu Bakr al-Baghdadi following the complete U.S. military withdrawal from Iraq on December 18, 2011.^[11] This departure created a security vacuum exacerbated by Prime Minister Nouri al-Maliki's Shiite-centric governance, which marginalized Sunni Arabs through de-Baathification policies, arbitrary arrests, and exclusion from power, fostering widespread resentment and enabling ISIS recruitment in Sunni-majority areas like Anbar province.^[11] By 2013, ISIS had expanded into Syria amid the civil war, breaking from al-Qaeda due to strategic disagreements and rebranding to reflect its cross-border ambitions.^[12] In early 2014, ISIS consolidated gains in Iraq, seizing Fallujah on January 4 and parts of Ramadi, marking its first major urban footholds since the U.S. withdrawal.^[13] The group's momentum accelerated with a coordinated offensive in June 2014, capturing Tikrit on June 11 and, most critically, Mosul—Iraq's second-largest city—between June 4 and 10. Despite numerical superiority, approximately 30,000 Iraqi security forces collapsed in disarray, abandoning positions and leaving behind U.S.-supplied equipment valued at over $1 billion, including helicopters, tanks, and artillery.^[2] ISIS fighters, numbering around 1,500 in the assault, also looted Mosul's central bank for an estimated $400 million to $500 million in cash, bolstering their finances and enabling further expansion toward Baghdad.^[13] These conquests demonstrated the fragility of Iraq's military, rooted in poor leadership, corruption, and low morale, while highlighting ISIS's tactical proficiency in exploiting tribal alliances and Sunni grievances.^[11] On June 29, 2014, ISIS formally declared the establishment of a caliphate across territories held in Iraq and Syria, renaming itself simply the Islamic State and proclaiming Baghdadi as its caliph, demanding allegiance from Muslims worldwide.^[13] This announcement followed swift territorial advances that threatened Iraq's capital and Kurdish regions, accompanied by systematic atrocities including mass executions and the displacement of minorities. The precipitating crisis intensified as ISIS approached Erbil and Baghdad, risking state collapse and humanitarian catastrophe, which prompted urgent Iraqi requests for U.S. assistance and set the stage for international military intervention.^[2] By mid-August, ISIS's execution of American journalist James Foley underscored the direct threat to Western interests, accelerating the shift from advisory support to airstrikes.^[12]

US Strategic Imperatives

The United States launched Operation Inherent Resolve on June 15, 2014, primarily to counter the Islamic State of Iraq and Syria (ISIS), which had seized Mosul on June 10, 2014, and threatened to overrun Baghdad, endangering U.S. allies and personnel in the region. This offensive followed ISIS's exploitation of the post-2011 U.S. withdrawal from Iraq, enabling the group to control territory spanning roughly 100,000 square kilometers by mid-2014, generate revenue from oil sales estimated at $1-3 million daily, and attract over 30,000 foreign fighters, creating a base for global terrorist operations.^[14]^[15] The core imperative was to degrade and ultimately destroy ISIS's caliphate to prevent it from serving as a launchpad for attacks on the U.S. homeland, as evidenced by ISIS-inspired plots and the beheadings of American journalists James Foley on August 19, 2014, and Steven Sotloff on September 2, 2014.^[16] A secondary but critical driver was protecting U.S. forces and partners, including an initial authorization for airstrikes on August 7, 2014, to safeguard approximately 40,000 Yazidis from genocide on Mount Sinjar and defend 30,000 U.S. personnel in Erbil against ISIS advances.^[15] At Iraq's government invitation, the U.S. aimed to enable Iraqi Security Forces (ISF) to reclaim sovereignty without committing large ground combat units, reflecting a strategy to avoid the protracted commitments of prior wars while addressing the power vacuum that allowed ISIS's rise.^[4]^[1] This "by, with, and through" partnering model sought to build local capacity—training over 240,000 ISF personnel since 2014—to ensure long-term stability and deny ISIS resurgence, thereby safeguarding U.S. interests in regional energy flows and countering Iranian influence without direct territorial control.^[4]^[17] Broader imperatives included upholding international norms against mass atrocities and maintaining U.S. credibility as a security guarantor, as ISIS's territorial control facilitated propaganda that inspired lone-actor attacks in the West, such as the 2015 San Bernardino shooting linked to ISIS ideology.^[18] The operation's focus on airpower, special operations, and coalition-building—initially with five Arab states joining U.S. strikes—aimed to minimize American casualties (fewer than 100 U.S. combat deaths by 2021) while disrupting ISIS's command, logistics, and finances, reducing its fighters from a peak of 30,000-35,000 to under 10,000 by 2019.^[14]^[19] This approach aligned with preventive self-defense under Article 51 of the UN Charter, prioritizing the elimination of ungoverned spaces that could export terrorism over indefinite occupation.^[15]

Legal Foundations Under International Law

The legal basis for Operation Inherent Resolve in Iraq rested on an explicit invitation from the Iraqi government, which requested international assistance to combat ISIS advances. On June 18, 2014, Iraqi Prime Minister Nouri al-Maliki formally appealed to the United States for airstrikes against ISIS militants, following their capture of Mosul and other territories, framing the request as essential to preserve Iraq's sovereignty and territorial integrity.^[20] This invitation aligned with customary international law permitting foreign military intervention at the behest of a recognized sovereign government facing internal threats, as affirmed in state practice and scholarly analysis of consent-based operations.^[21] Subsequent Iraqi leaders, including Prime Minister Haider al-Abadi, reaffirmed and expanded this consent, enabling coalition advisory, training, and kinetic support to Iraqi security forces without constituting an occupation or violation of sovereignty.^[22] In Syria, the absence of consent from the Assad government complicated the legal framework, prompting the United States to invoke collective self-defense under Article 51 of the UN Charter on behalf of Iraq and regional partners. The U.S. State Department asserted that ISIS's territorial control in Syria facilitated cross-border attacks into Iraq, constituting an ongoing armed threat that justified proportionate strikes to degrade safe havens and prevent further incursions, without requiring Syrian approval due to the regime's unwillingness or inability to address the threat.^[23] This rationale extended post-9/11 precedents for self-defense against non-state actors exercising de facto territorial authority, as ISIS governed significant areas with administrative structures akin to a quasi-state entity.^[24] Coalition partners, including the United Kingdom and Australia, echoed this justification, conducting operations in Syria from September 2014 onward under similar interpretations of necessity and proportionality.^[25] United Nations Security Council resolutions provided supplementary normative support, though not explicit use-of-force authorization under Chapter VII. Resolution 2170 (August 15, 2014) condemned ISIS as a threat to international peace and obligated states to suppress its recruitment and financing, while Resolution 2249 (November 20, 2015) urged member states to adopt "all necessary measures" to eradicate ISIS havens in Syria and Iraq, responding to attacks like those in Paris.^[26] These measures reinforced the coalition's actions amid veto threats from Russia, which blocked broader mandates, but reflected consensus on ISIS's global threat status without altering the primary reliance on self-defense and invitation.^[27] Critics, including some international legal scholars, have contested the extension of Article 51 to unconsented territorial incursions against non-state actors, arguing it risks eroding sovereignty norms, yet widespread state participation in the coalition evidenced emerging acceptance in practice.^[28]

Objectives and Operational Strategy

Core Military and Political Goals

The core military objective of Operation Inherent Resolve, launched on September 16, 2014, was to militarily defeat the Islamic State of Iraq and Syria (ISIS), also known as Daesh or ISIL, by dismantling its self-proclaimed caliphate across Iraq and Syria through airstrikes, ground enablement of partner forces, and targeted operations against ISIS leadership and infrastructure. This encompassed degrading ISIS's combat capabilities, disrupting its command and control, and supporting Iraqi Security Forces (ISF) and Syrian partner forces in recapturing over 110,000 square kilometers of territory held by ISIS by March 2019, including key cities like Mosul and Raqqa.^[2]^[1] The U.S. Department of Defense articulated this as a comprehensive counterterrorism strategy to "degrade, and ultimately destroy" ISIS, prioritizing the elimination of safe havens that could enable external attacks.^[29] Politically, the operation aimed to enable conditions for regional stability by empowering local partners to govern recaptured areas, rebuild institutions, and prevent ISIS resurgence through non-combat lines of effort such as governance support, economic stabilization, and countering violent extremism. This included bolstering the sovereignty of the Iraqi government against territorial threats and facilitating the return of over 5 million internally displaced persons by fostering inclusive security and political processes, though challenges persisted due to sectarian divisions and Iranian influence in Iraq.^[30]^[17] The strategy emphasized "by, with, and through" local forces to build sustainable capacities, avoiding large-scale U.S. ground combat deployments while aligning with broader U.S. imperatives to counter global jihadist networks.^[31]

By-With-Through Partnering Model

The "by, with, and through" (BWT) partnering model emerged as the central operational framework for Operation Inherent Resolve, emphasizing U.S. enablement of local partner forces to conduct ground combat against the Islamic State of Iraq and Syria (ISIS) while minimizing direct U.S. troop involvement in maneuver operations.^[32] This approach, formalized by U.S. Central Command (USCENTCOM), defined operations as partner-led, with U.S. forces providing advisory, assistive, and enabling support to address capability gaps in areas such as intelligence, logistics, and fire support.^[31] Adopted from 2014 onward, it reflected post-Iraq and Afghanistan lessons, prioritizing partner ownership to enhance sustainability and reduce U.S. casualties, though it required balancing partner reliability with U.S. operational control.^[19] The model's three components delineated distinct roles: "by" entailed limited U.S.-led actions, such as special operations raids or unilateral airstrikes when partners could not act; "with" involved close collaboration, including U.S. advisors embedded with partner units for planning and execution, as seen in the training of over 150,000 Iraqi Security Forces (ISF) personnel by U.S. advisors between 2014 and 2017; and "through" focused on indirect enablement, like precision airstrikes coordinated via joint operations centers, which delivered over 134,000 munitions against ISIS targets by March 2019.^[33] In Iraq, this manifested through advising ISF and Kurdish Peshmerga units during the 2016-2017 Mosul campaign, where U.S. support facilitated the recapture of the city after nine months of fighting led primarily by local forces.^[34] In Syria, BWT enabled the Syrian Democratic Forces (SDF)—a Kurdish-led coalition—to liberate Raqqa in 2017, with U.S. air and artillery providing decisive fire support amid urban combat.^[35] Implementation relied on U.S. ground contributions, including approximately 5,000 advisors and special operations personnel at peak deployment, who conducted security force assistance through training academies and forward operating bases, while coalition air assets—totaling over 30,000 sorties by 2018—integrated with partner targeting.^[19] This model achieved ISIS's territorial caliphate defeat by March 2019, with partners reclaiming over 110,000 square kilometers, but exposed challenges like partner force cohesion issues, as evidenced by ISF collapses in 2014 and SDF vulnerabilities to Turkish incursions post-2019.^[31]^[36] Despite these, analyses from the U.S. Army and RAND deemed BWT effective for rapid territorial gains with fewer than 100 U.S. combat deaths attributed to ground operations, though it underscored dependencies on sustained U.S. enablers for enduring stability.^[37]^[19] By 2021, the approach transitioned to a "new phase" of advising and capacity-building, with U.S. forces reduced to under 2,500 in Iraq and 900 in Syria, focusing on counter-terrorism rather than large-scale enablement.^[38]

Evolving Rules of Engagement and Tactics

Initial rules of engagement (ROE) for U.S. forces in Operation Inherent Resolve emphasized minimizing direct combat involvement, prohibiting advisers from accompanying Iraqi Security Forces (ISF) into battle, and requiring high-level approval for strikes to limit collateral damage and U.S. casualties.^[34] Tactics focused on airpower dominance, with over 20,000 coalition airstrikes by mid-2015 targeting ISIS command nodes, oil infrastructure, and convoys, supported by intelligence, surveillance, and reconnaissance (ISR) assets rather than ground maneuvers.^[39] This "by, with, and through" model relied on enabling partner forces like the ISF and Peshmerga through rear-area advising at division headquarters, avoiding large-scale U.S. troop deployments beyond special operations raids, such as the October 2015 capture of an ISIS oil minister.^[40] By late 2016, ROE evolved under Tactical Directive #1, issued December 22, 2016, permitting U.S. advisers to position at the last covered and concealed spot behind the forward line of own troops (FLOT), enabling real-time authorization of surface and joint fires to accelerate ISF advances.^[34] Tactics shifted toward brigade- and battalion-level advising, integrating close combat air support (e.g., AH-64 Apache helicopters) and artillery (e.g., M109A6 Paladins, HIMARS systems) during operations like the Battle of Mosul, where U.S. forces brokered Arab-Kurd coordination and provided $415 million in funding for staging over 100,000 ISF troops.^[34] Special operations tactics adapted to ISIS urban defenses, countering vehicle-borne improvised explosive devices (VBIEDs), snipers, tunnels, and drones through manned-unmanned teaming and psychological operations, reducing SVBIED threats by over 80% in eastern Mosul by January 2017.^[40]^[34] In 2017, the Trump administration further loosened ROE by delegating targeting authority to field commanders, bypassing lengthy White House reviews that had delayed strikes under prior policy, which accelerated dynamic targeting against ISIS in Raqqa and Mosul's western districts.^[41] This enabled more responsive close air support and artillery barrages, contributing to the territorial collapse of the ISIS caliphate by March 2019, though it correlated with a reported rise in civilian casualties from 89 in 2016 to 324 in the first six months of 2017 per Airwars monitoring.^[42] Tactics intensified with multi-axis partner offensives, enhanced counter-drone systems (e.g., AUDS), and tank integrations to dismantle ISIS industrial bases in urban fights, liberating key areas like western Mosul by July 10, 2017, despite high ISF losses (e.g., ~500 Iraqi Counterterrorism Service killed, 3,000 wounded).^[34] Post-caliphate, ROE tightened for enduring defeat phases, emphasizing advisory transitions to partners amid force drawdowns.^[4]

Phase	Key ROE Change	Tactical Adaptation	Impact
2014–2015	Rear-area advising only; general officer approval for fires	Air-centric strikes; limited SOF raids	Slowed partner gains in Anbar; ~70 Iraqi cities liberated by mid-2017 with advisory support^[40]
2016	Tactical Directive #1: Forward positioning for fire authorization	Brigade-level enabling; CCA integration	Enabled Mosul encirclement; faster clearance in Fallujah (June 2016) vs. Ramadi^[34]
2017–2019	Delegated authority to commanders	Dynamic targeting; counter-VBIED/drone ops	Caliphate defeat; higher civilian risks noted by observers^[41]^[42]

Coalition Composition

Participating Nations and Commitments

The Global Coalition to Defeat ISIS, announced by the United States on September 10, 2014, comprises 89 member nations and international organizations, including commitments from Africa (e.g., Egypt, Nigeria, Tunisia), the Americas (Canada, United States), Asia-Pacific (Australia, Japan, New Zealand), Europe (Belgium, Denmark, France, Germany, Italy, Netherlands, United Kingdom), the Middle East (Bahrain, Iraq, Jordan, Kuwait, Saudi Arabia, United Arab Emirates), and institutions such as NATO, the European Union, and INTERPOL.^[43] These entities pledged multifaceted support, encompassing military operations, counter-financing measures, disruption of foreign fighter flows, and stabilization initiatives in Iraq, Syria, and globally, though the extent of tangible contributions differed widely among members.^[22] Military participation under Combined Joint Task Force - Operation Inherent Resolve (CJTF-OIR), formally established on October 17, 2014, was spearheaded by the United States, which supplied the majority of airstrikes (over 70% of coalition total), intelligence, special operations forces, and advisory personnel to enable partner ground forces.^[2] ^[14] A subset of coalition partners provided direct combat support, including airstrikes and limited ground elements, while others focused on training Iraqi and Syrian partner forces, logistics, and capacity-building without engaging in direct combat roles. Key military contributors included:

United Kingdom: Conducted hundreds of airstrikes, deployed special forces for advisory and reconnaissance missions, and provided training teams in Iraq, with commitments peaking at around 800 personnel by 2015.^[44]
France: Executed airstrikes via Operation Chammal, contributed special operations forces, and trained Iraqi troops, sustaining involvement through 2024 with naval and air assets.^[44]
Australia: Under Operation Okra, delivered airstrikes, deployed up to 780 personnel for training and advising, and provided special forces until ceasing operations in December 2024.^[45] ^[44]
Canada: Performed airstrikes with CF-18 fighters, sent special forces to support Kurdish forces, and trained over 6,000 Iraqi personnel before withdrawing combat elements in 2016 while continuing advisory roles.^[44]
Denmark and Belgium: Contributed F-16 airstrikes in both Iraq and Syria, with Denmark deploying up to 400 troops for training and Belgium focusing on limited air operations.^[44]
Netherlands: Flew F-16 airstrikes and provided training instructors for Iraqi forces.^[44]
Jordan, United Arab Emirates, and Saudi Arabia: Arab coalition members conducted airstrikes primarily in Syria starting September 23, 2014, with Jordan hosting coalition operations and UAE pilots participating in early strikes.^[44]
Germany and Italy: Emphasized non-combat roles, deploying trainers (Germany up to 1,100 advisors in Iraq by 2017) and equipment to build Iraqi security force capacities under framework nation agreements.^[46]

These commitments evolved over time, with many European and Australian forces shifting to advisory and training missions post-2017 as ISIS lost territorial control, reflecting the coalition's "by, with, and through" approach prioritizing local partners.^[4] By 2025, operations in Iraq transitioned toward bilateral U.S.-Iraq arrangements, with reduced coalition footprints amid ongoing defeat-ISIS efforts in Syria.^[47]

Command Structure and Coordination

Combined Joint Task Force – Operation Inherent Resolve (CJTF-OIR) serves as the central command entity for the multinational coalition conducting military operations against ISIS in Iraq and Syria. Established by the U.S. Department of Defense on October 17, 2014, CJTF-OIR formalized the integration of U.S. and partner nation forces previously operating under U.S. Central Command (CENTCOM) direction.^[2] The task force operates as the military arm of the Global Coalition to Defeat ISIS, encompassing 78 nations and five international organizations committed to degrading and defeating the terrorist group. CJTF-OIR falls under the operational control of CENTCOM, with its commander reporting to the CENTCOM commander for strategic oversight.^[48] The structure employs a joint task force model, incorporating U.S. service components for air, maritime, and land operations, while integrating coalition partners through assigned liaison officers and joint staff positions. Initially, U.S. Army Central (ARCENT) functioned as the Joint Force Land Component Command (JFLCC), managing ground enablement until the full CJTF transition enhanced multinational integration.^[49] This evolution facilitated decentralized mission command, enabling rapid coordination across distributed forces in Iraq and Syria. Leadership of CJTF-OIR is U.S.-led, with the current commander being U.S. Army Brigadier General Kevin J. Lambert, who assumed command on July 22, 2025, from Major General Kevin C. Leahy.^[50] Supporting roles include a U.S. Command Sergeant Major and coalition deputies, such as a British brigadier for operations coordination.^[51] The chief of staff, Brigadier General Leslie F. Hauck III, oversees staff functions including intelligence, logistics, and planning.^[51] Coordination emphasizes a "by, with, and through" approach, advising and assisting local partner forces like the Iraqi Security Forces and Syrian Democratic Forces without direct U.S. combat involvement post-2014 guidelines.^[4] Mechanisms include key leader engagements for trust-building, joint operational planning, and synchronized effects across air, ground, and special operations domains.^[52] Military information support operations (MISO) are planned per joint doctrine and coordinated with coalition partners and host nations to align messaging and effects.^[53] This structure has sustained coalition interoperability, with rotations of commands like III Corps historically providing headquarters support until 2018.^[54]

Deployed Forces and Assets

Air, Naval, and Intelligence Capabilities

The air component of Operation Inherent Resolve relied heavily on U.S. Air Force and coalition fixed-wing aircraft, remotely piloted aircraft, and rotary-wing assets to deliver precision strikes against ISIS targets in Iraq and Syria.^[55] U.S. forces deployed F-22 Raptors from the 27th Fighter Squadron, supporting over 250 air tasking orders during deployments starting in 2017.^[56] Electronic warfare aircraft from the 386th Air Expeditionary Wing enhanced strike effectiveness by jamming enemy communications and radars.^[57] Coalition partners, including the French Air Force, contributed fighters, tankers, and airborne early warning platforms to integrate with the Combined Air Operations Center (CAOC).^[58] Naval capabilities supported the operation through carrier strike groups operating in the Arabian Gulf and eastern Mediterranean Sea, launching carrier-based airstrikes and cruise missiles.^[59] The USS Nimitz Carrier Strike Group commenced air operations from the Arabian Gulf on September 18, 2020, contributing to maritime security and OIR missions.^[60] Similarly, the George H.W. Bush Carrier Strike Group initiated strikes from the Mediterranean in February 2017, while the Abraham Lincoln and Eisenhower groups provided sustained presence and sorties in subsequent years.^[59]^[61]^[62] The French carrier Charles de Gaulle also joined with its strike group, including frigates and submarines, augmenting naval airpower.^[63] Intelligence capabilities encompassed intelligence, surveillance, and reconnaissance (ISR) assets, fusion centers, and multinational sharing mechanisms to identify and target ISIS leadership and infrastructure.^[1] U.S. and coalition forces provided airborne ISR, including drones and manned platforms, to support ground partners and enable dynamic targeting. Intelligence sharing through bilateral channels and the Defeat-ISIS mission's counter-terrorism frameworks disrupted foreign fighter flows and financing networks.^[64]^[65] Iraqi partners developed capabilities to re-analyze intelligence products, with coalition advisors enhancing joint operations centers for real-time data fusion.^[1]

Ground Advisors, Special Operations, and Logistics

United States ground advisors, primarily from the Army and Marine Corps, were deployed to Iraq beginning in June 2014 to train, advise, and assist Iraqi Security Forces (ISF) and Kurdish Peshmerga units in countering ISIS advances, operating under strict rules limiting direct combat engagement while enabling partner-led operations.^[40] By late 2017, U.S. ground forces, including advisors, numbered approximately 2,000 in Iraq, focusing on building partner capacity through embedded training teams that provided tactical advice during major offensives such as the battles for Ramadi, Fallujah, and Mosul.^[40] ^[19] In Syria, advisor presence was smaller and more targeted, with about 50 U.S. Special Operations Forces (SOF) advisors supporting Syrian Democratic Forces (SDF) by December 2015, emphasizing counter-IED training, force protection, and operational planning for assaults on Raqqa and Baghouz.^[40] ^[4] Since 2014, coalition advisors have trained over 240,000 ISF personnel, facilitating the transfer of eight bases to Iraqi control by 2020 as partners assumed greater operational independence.^[4] Special Operations Forces, coordinated under the Special Operations Joint Task Force - Operation Inherent Resolve (SOJTF-OIR) established in January 2016, conducted high-risk missions including raids, intelligence-driven strikes, and direct action against ISIS leadership and infrastructure.^[40] Key activities encompassed a 2015 raid on the ISIS "oil minister's" compound, an October 22, 2015, assault on an ISIS prison, seizure of Al-Tanf garrison in March 2016 to disrupt supply lines, and the October 26, 2019, raid killing ISIS leader Abu Bakr al-Baghdadi.^[40] Army SOF also advised during SDF offensives, contributing to the liberation of 35,000 square miles in Syria and 70 cities in Iraq by mid-2017, while suffering seven of the 13 U.S. Army fatalities in OIR from 2014 to 2019.^[40] These forces enabled partner "finish" operations, such as SDF unilateral arrests of ISIS operatives, through intelligence sharing and joint patrols, aligning with the "by, with, and through" partnering model to minimize U.S. footprint.^[4]^[40] Logistics support for OIR involved sustainment of coalition advisors and SOF through regional support groups, engineer units, and air base operations at key sites like Al Asad Air Base in Iraq, where National Guard elements such as the 347th Regional Support Group provided base life support and convoy security starting in 2022.^[66] Coalition engineers, including the 416th Theater Engineer Command, fortified bases in northeast Syria with T-walls and defenses as of April 2025 to counter ISIS threats.^[67] The Counter-ISIS Train and Equip Fund allocated over $5 billion for equipping ISF, including logistics enablers like maintenance and supply chains, enabling sustained partner operations without large-scale U.S. conventional logistics deployments.^[4] By 2021, forward headquarters elements relocated from Iraq to Kuwait, such as at Al Asad, to streamline support amid transitions to enduring defeat missions.^[68]

Operational History

2014: Launch and Initial Airstrikes

United States Central Command initiated airstrikes against Islamic State forces in Iraq on August 8, 2014, targeting artillery units shelling Kurdish positions near Erbil and convoys advancing on the city, as well as militants besieging Yazidi civilians on Mount Sinjar.^[15] These operations employed U.S. Air Force, Navy, and Marine Corps aircraft, including F-16s, F/A-18s, AV-8B Harriers, and MQ-9 Reapers, alongside cruise missiles launched from naval assets in the Arabian Gulf and Red Sea.^[15] The strikes aimed to protect U.S. diplomatic personnel and facilities in Erbil, disrupt Islamic State supply lines, and enable humanitarian resupply efforts to trapped populations.^[15] On September 22, 2014, President Barack Obama authorized the expansion of airstrikes into Syria to prevent the Islamic State from using the country as a safe haven for planning attacks.^[69] The first strikes occurred on September 23, involving over 180 aircraft and more than 200 munitions hitting 12 Islamic State targets near Raqqa, including oil refineries, storage facilities, and fighter positions, as well as strikes against Khorasan Group elements affiliated with al-Qaeda.^[70] Five Arab states—Bahrain, Jordan, Qatar, Saudi Arabia, and the United Arab Emirates—participated in these initial Syrian operations by providing aircraft for strikes or support roles.^[70] The Department of Defense formally established Combined Joint Task Force – Operation Inherent Resolve on October 15, 2014, under U.S. Army Central Command, to oversee and coordinate the growing coalition air campaign and advisory efforts without deploying large-scale U.S. ground combat units.^[2] Early strikes destroyed hundreds of Islamic State vehicles, weapons caches, and command nodes, halting the group's momentum in northern Iraq and contributing to the relief of the Sinjar siege by late December 2014.^[14] U.S. forces reported no combat losses in these initial phases, while estimating hundreds of Islamic State fighters killed, though independent verification of enemy casualties remained limited.^[55] Coalition reports indicated minimal civilian casualties from the 2014 strikes, with subsequent investigations confirming isolated incidents amid the emphasis on precision targeting near populated areas.

2015: Ground Partner Enablement

In 2015, Operation Inherent Resolve emphasized the "by, with, and through" model to enable indigenous ground forces in Iraq and Syria, deploying U.S. military advisors and trainers to build partner capacity amid setbacks like the fall of Ramadi to ISIS in May.^[34] Approximately 3,500 U.S. trainers operated across six bases, including Camp Taji, where Task Force Panther instructed around 12,400 Iraqi soldiers in urban combat, marksmanship, breaching, explosive ordnance disposal, and medical techniques over nine months.^[34] This effort, funded by $1.6 billion from the Iraq Train and Equip Fund in fiscal year 2015, provided equipment such as 2,000 antiarmor rockets and 30 mine-resistant vehicles to enhance Iraqi Security Forces' (ISF) combat engineering and mobility.^[34] U.S. advisors embedded at division and brigade levels coordinated with ISF units, including the Counter-Terrorism Service (CTS, or Golden Division), to support offensives against ISIS strongholds.^[34] In June, around 450 troops from Task Force Taqaddum advised the ISF's 8th, 10th, and 16th Divisions near Al-Taqaddum Air Base for the Ramadi campaign, though initial remote advising from headquarters limited tactical influence due to policy restrictions barring entry into combat zones.^[34] By October, training expanded to approximately 5,000 Sunni militia fighters, with 2,500 properly equipped, enabling the ISF to retake Ramadi's city center in December with CTS leading assaults supported by U.S. intelligence, surveillance, reconnaissance, and 292 coalition airstrikes since July.^[34] Kurdish Peshmerga forces, advised indirectly through coalition channels, advanced against ISIS west of Kirkuk starting September 30 and received airstrike support near Sinjar on November 11, reclaiming terrain and degrading ISIS logistics.^[71]^[72] In Syria, U.S. special operations forces initiated limited deployments in November to advise and assist emerging partners like the Syrian Democratic Forces (SDF), formed in October primarily from Kurdish YPG units, focusing on intelligence sharing and enabling small-scale operations rather than large-scale training.^[40] Challenges persisted, including ISF sectarian divisions favoring Shi'a militias, high attrition in elite units like CTS (around 50% casualties), and ISIS tactics such as vehicle-borne improvised explosive devices, which slowed progress and highlighted the need for forward advising adjustments by late 2015.^[34] By year's end, the campaign shifted to Phase II counterattacks, with ground enablement laying groundwork for major 2016 offensives despite uneven partner performance.^[2]

2016: Major Offensives Begin

In Iraq, Iraqi Security Forces continued clearing operations in Ramadi into early 2016 following its recapture from ISIS control in December 2015, methodically searching for improvised explosive devices and remaining fighters amid thousands of booby-trapped structures. By January, coalition advisors embedded with Iraqi units facilitated the demolition of over 1,000 explosive-rigged buildings, enabling the safe return of displaced civilians and marking a turning point in Anbar Province stabilization. This progress shifted focus to Fallujah, where Iraqi army, federal police, and counterterrorism units launched an offensive on May 23, 2016, supported by coalition airstrikes targeting ISIS command posts, weapon caches, and fleeing convoys.^[73] The operation culminated on June 26, 2016, when Iraqi forces declared Fallujah fully liberated after clearing the last ISIS-held neighborhoods, with coalition strikes destroying approximately 175 ISIS vehicles in two separate convoys attempting to evacuate fighters.^[74]^[75] These Anbar successes, bolstered by over 2,000 coalition airstrikes in the province since 2014, reduced ISIS territorial control in western Iraq by more than 50% by mid-year.^[76] Preparations for the larger Battle of Mosul intensified throughout 2016, involving training and equipping over 90,000 Iraqi troops and Peshmerga fighters at coalition tactical assembly areas.^[77] On October 17, 2016, Iraqi Security Forces initiated a multi-axis offensive from multiple directions, advancing toward ISIS's de facto capital with immediate coalition support including artillery fire missions and intelligence sharing.^[77] In the opening days, coalition aircraft conducted around 100 airstrikes against ISIS tactical units, bridges, and supply lines, enabling initial gains of up to 20 kilometers in some sectors despite ISIS counterattacks using car bombs and snipers. U.S. special operations forces provided on-the-ground advising to limit civilian casualties through precise targeting, though the urban environment posed significant challenges with ISIS embedding among 1.5 million residents.^[77] In Syria, the Syrian Democratic Forces (SDF), a Kurdish-led coalition partner, launched the Manbij offensive on May 31, 2016, aiming to sever ISIS supply lines from Turkey to Raqqa.^[78] Supported by U.S. special operations advisors and coalition airstrikes from MQ-9 Reapers and other platforms, SDF fighters encircled and isolated Manbij by mid-June, methodically clearing ISIS defenses in house-to-house fighting.^[79] The city was declared liberated on August 13, 2016, after two months of combat that killed hundreds of ISIS fighters and disrupted key logistics routes, though ISIS tactics included using civilians as human shields during retreats.^[80]^[81] This victory expanded SDF-controlled territory along the Euphrates, positioning forces for subsequent advances toward Raqqa and contributing to ISIS territorial losses exceeding 40% in Syria by year's end.^[82] Overall, 2016 airstrikes under Operation Inherent Resolve totaled over 15,000 strikes across Iraq and Syria, a decline from peak 2015 levels but increasingly focused on close air support for ground maneuvers, resulting in the elimination of thousands of ISIS combatants and destruction of heavy equipment.^[83] These offensives demonstrated the efficacy of coalition-enabled partner forces in reclaiming urban areas, though persistent issues like ISIS guerrilla tactics and sectarian tensions in Iraq complicated post-liberation governance.^[73]

2017: Caliphate Peaks and Declines

In early 2017, the Islamic State of Iraq and Syria (ISIS) retained control over its self-proclaimed territorial caliphate's core urban centers, including Mosul in Iraq—its largest Iraqi stronghold—and Raqqa in Syria, which served as the group's de facto capital since 2014.^[84] ^[85] These holdings represented the sustained peak of ISIS's physical caliphate footprint amid ongoing offensives, with the group controlling approximately 40,000 square miles across Iraq and Syria at the start of the year.^[86] The Combined Joint Task Force–Operation Inherent Resolve (CJTF-OIR) intensified support for partner ground forces, conducting airstrikes, providing intelligence, and enabling operations that accelerated ISIS's territorial contraction. In Iraq, Iraqi Security Forces (ISF), backed by coalition advisors and airpower, completed the nine-month Battle of Mosul, entering the city's western districts in January and declaring full liberation on July 10, 2017, after intense urban fighting that displaced over 1 million civilians and resulted in thousands of ISIS fighter casualties. ^[87] Subsequent ISF operations liberated Tal Afar in August 2017 and Hawijah in October, further eroding ISIS's hold in northern and western Iraq.^[86] In Syria, the Syrian Democratic Forces (SDF)—a Kurdish-led, U.S.-partnered militia—launched the Raqqa offensive on June 6, 2017, encircling the city by early July and systematically clearing ISIS defenses with coalition close air support exceeding 2,000 strikes during the campaign.^[85] The SDF announced Raqqa's liberation on October 20, 2017, after four months of combat that killed an estimated 3,000-4,000 ISIS fighters and uncovered mass graves indicative of the group's brutal governance.^[85] ^[88] By December 2017, these gains had reduced ISIS-controlled territory by over 60 percent to less than 15,000 square miles, with Iraq formally declaring victory over the caliphate on December 9 despite residual pockets.^[86] ^[2] CJTF-OIR transitioned leadership to Lt. Gen. Paul Funk II in September, emphasizing by-with-and-through partner enablement to degrade ISIS's command structure and prevent resurgence, though the group adapted by dispersing into insurgent cells.^[2]

2018: Territorial Contractions

In 2018, the Combined Joint Task Force–Operation Inherent Resolve (CJTF-OIR) and partner forces focused on eliminating the Islamic State of Iraq and Syria's (ISIS) remaining territorial holdings, reducing the group's control to isolated pockets primarily in eastern Syria's Middle Euphrates River Valley. By early 2018, ISIS had already lost approximately 95 percent of its peak territory from 2014–2017, with Iraqi Security Forces (ISF) declaring victory over the group in Iraq by December 2017; however, persistent operations targeted residual fighters in rural and desert areas to prevent regrouping.^[13]^[89] In Syria, the Syrian Democratic Forces (SDF), supported by coalition airstrikes and advisors, conducted ground offensives that contracted ISIS holdings to an estimated one percent of Syrian territory by September 2018, centered around Hajin and adjacent villages.^[90] Iraqi forces prioritized clearing ISIS remnants in western Iraq, including the Anbar desert and Euphrates River Valley, through joint operations with coalition advisors that involved raids, intelligence-driven strikes, and securing border areas to block ISIS infiltration from Syria. These efforts dismantled ISIS command nodes, weapon caches, and safe houses, with ISF pursuing fighters in small pockets across governorates like Nineveh and Salah ad-Din, preventing the group from establishing sustainable insurgent bases.^[91] By late 2018, Iraq had regained full territorial control, though low-level threats persisted, necessitating ongoing CJTF-OIR support for stabilization.^[90] In Syria, the SDF's campaign in Deir ez-Zor province accelerated territorial contractions, with advances reclaiming villages and oil fields from ISIS defenses fortified with improvised explosives and foreign fighters. On September 10, 2018, the SDF announced an offensive to reclaim the last ISIS-held areas along the Iraq-Syria border, initiating what became known as Operation Roundup, which involved systematic clearing of Hajin and surrounding positions.^[92] Coalition forces provided close air support, conducting thousands of strikes that destroyed ISIS vehicles, fighting positions, and leadership elements, resulting in the deaths of thousands of fighters over the year.^[93] These operations reduced ISIS to a shrinking enclave by year's end, though the group mounted fierce resistance with counterattacks and civilian human shielding tactics.^[93]

2019: Geographic Defeat of ISIS

In early 2019, Combined Joint Task Force – Operation Inherent Resolve (CJTF-OIR) intensified support for the Syrian Democratic Forces (SDF) in Operation Roundup to clear ISIS remnants from the Middle Euphrates River Valley, the last major holdout following territorial losses in 2018. Coalition forces provided precision airstrikes, intelligence sharing, and logistical assistance to enable SDF ground advances against ISIS fighters entrenched in Hajin and surrounding villages, where the group had concentrated foreign fighters, heavy weapons, and improvised explosive devices. By February, SDF operations had compressed ISIS into a shrinking pocket around Baghouz Fawqani, prompting mass surrenders among ISIS supporters, including an estimated 60,000 civilians and combatants who emerged from the enclave over several weeks.^[94] The decisive phase unfolded in March, with SDF forces launching assaults on Baghouz, ISIS's final territorial bastion, amid fierce resistance involving booby-trapped structures and counterattacks. Coalition airstrikes targeted ISIS command posts, vehicle convoys, and fighting positions, contributing to the elimination of hundreds of militants during the offensive; for instance, on March 12, SDF reported killing dozens of ISIS fighters in clashes near the village. U.S. and partner aircraft flew over 200 strikes in the first quarter of 2019 alone, degrading ISIS capabilities while SDF ground elements cleared remaining defenses.^[95] On March 23, 2019, U.S. Central Command Commander General Joseph L. Votel announced the liberation of ISIS's last sliver of territory in Baghouz, marking the geographic defeat of the self-proclaimed caliphate that had once controlled over 100,000 square kilometers across Iraq and Syria. This followed the SDF's capture of the village, ending ISIS's ability to hold conventional territory after a four-year campaign that saw the group lose 100 percent of its urban centers and populated areas. CJTF-OIR emphasized that the achievement resulted from partner-led ground efforts enabled by coalition enablers, with no U.S. conventional ground combat troops directly engaged.^[96]^[94]

2020-2022: Shift to Defeat-ISIS Mission

Following the territorial defeat of the Islamic State's (ISIS) self-proclaimed caliphate in March 2019, Operation Inherent Resolve (OIR) transitioned in 2020 toward an enduring "defeat-ISIS" posture, emphasizing prevention of ISIS resurgence through partner enablement, intelligence sharing, and targeted strikes rather than large-scale territorial campaigns. Coalition forces, led by the United States, focused on building capacity among Iraqi Security Forces (ISF) and Syrian Democratic Forces (SDF) via advising, training, and logistics support, while conducting unilateral operations to disrupt ISIS networks. In Iraq, U.S. personnel numbers stood at approximately 5,200 combat advisors at the start of 2020, supporting ISF-led raids that resulted in the capture or elimination of over 1,000 ISIS operatives annually. In Syria, around 900 U.S. troops maintained presence in eastern oil fields to deny ISIS revenue and enable SDF operations, with coalition airstrikes dropping to fewer than 100 per year but remaining precise against high-value targets.^[97] In February 2021, President Biden announced a phased reduction of U.S. forces in Iraq to 2,500 by September 2021, marking the end of the combat mission under OIR and a shift to a bilateral U.S.-Iraq advisory partnership focused on counter-ISIS efforts, while retaining authority for unilateral actions against ISIS threats. This drawdown relocated some capabilities to Kuwait but preserved operational tempo, with coalition partners conducting over 200 partner-enabled operations in Iraq that year, killing or detaining hundreds of ISIS fighters. In Syria, U.S. forces faced heightened risks from Iranian-backed militia attacks, prompting defensive responses including airstrikes on October 20, 2021, that neutralized over 40 militants following rocket assaults on U.S. positions. The transition underscored a causal emphasis on local ownership to sustain gains, as empirical data from prior phases showed partner-led forces were more effective long-term in counterinsurgency than direct U.S. intervention.^[98] By 2022, OIR issued a new campaign plan in January to guide the defeat-ISIS mission, prioritizing counter-network operations, leadership decapitation, and disruption of ISIS financing and propaganda. U.S. Central Command (CENTCOM) and partners executed hundreds of operations across Iraq and Syria, including ISF-led raids that eliminated key ISIS figures such as the group's Iraq operations chief in February 2022. Airstrikes and special operations forces targeted ISIS sleeper cells, with coalition reports confirming over 150 ISIS fighters killed in the first half of the year alone. In Syria, SDF operations, supported by U.S. intelligence and logistics, cleared ISIS pockets in the Badia desert, capturing foreign fighters and preventing regrouping. This phase reflected a realistic assessment that ISIS persisted as a decentralized insurgency capable of asymmetric attacks, necessitating sustained but reduced U.S. enablers to avoid territorial vacuums exploited in prior conflicts.^[99]^[97]^[100]

2023-2025: Stabilization, Transitions, and Enduring Operations

In 2023, Combined Joint Task Force - Operation Inherent Resolve (CJTF-OIR) emphasized stabilization efforts through advisory support to Iraqi Security Forces (ISF) and partner forces in Syria, conducting 48 defeat-ISIS operations in February alone that resulted in 22 ISIS fighters killed and multiple detentions.^[101] In May, coalition and partner forces executed 38 operations, targeting ISIS leadership and infrastructure to prevent resurgence amid rising attack attempts.^[102] These activities aligned with a post-territorial defeat phase, prioritizing capacity building and intelligence-driven strikes over large-scale offensives.^[103] A key transition occurred in Iraq, where the U.S. and Iraqi governments announced on September 27, 2024, a two-phase plan to end the coalition's military mission under OIR and shift to a bilateral security partnership.^[47] Phase one involved closing all coalition sites in Iraq by September 2025, with U.S. forces consolidating in Erbil and redirecting efforts toward Syria to counter ISIS cross-border threats.^[104] By October 2025, the drawdown commenced, reducing U.S. troop presence while maintaining advisory roles to bolster ISF capabilities against ISIS remnants, reflecting Iraq's improved sovereignty and reduced operational dependency on coalition bases.^[105] In Syria, enduring operations persisted with support to the Syrian Democratic Forces (SDF), including strikes against ISIS targets and destruction of unmanned aerial systems, as seen in weekly CENTCOM updates through late 2024.^[106] By April 2025, the U.S. consolidated forces from approximately 2,000 to under 1,000 at select bases, enhancing efficiency amid SDF-led counter-ISIS raids that detained fighters and seized weapons caches.^[107] These efforts addressed ISIS reconstitution attempts, with partner forces conducting operations near Dayr az-Zawr in early 2025 despite regional tensions.^[108] Throughout 2023-2025, CJTF-OIR's focus on stabilization included leadership eliminations, such as a May 2023 strike on an Al Qaeda figure in northwest Syria, and network disruptions to degrade ISIS operational capacity.^[109] Quarterly assessments noted sustained partner training and equipment provision, aiming for long-term deterrence against ISIS attacks, which CENTCOM reported were on pace to double in 2024 before coalition interventions curbed momentum.^[1] Command changes, including the August 2023 handover and August 2024 transition, underscored operational continuity in this enduring mission.^[110]^[52]

Key Engagements and Sub-Operations

Battle of Mosul

The Battle of Mosul (2016–2017) was a major offensive by Iraqi security forces (ISF), backed by U.S.-led coalition airpower, intelligence, and advisors under Operation Inherent Resolve, to recapture Iraq's second-largest city from ISIS control, which the group had seized on 10 June 2014. Approximately 94,000 ISF personnel, including Iraqi Army divisions, Counter-Terrorism Service (CTS) units, and Federal Police, alongside Kurdish Peshmerga and select Popular Mobilization Forces (PMF) militias, encircled and assaulted the city against 5,000–12,000 ISIS fighters who had fortified positions over two years with extensive booby traps, tunnels, and stockpiles of weapons.^[111]^[112] The campaign highlighted the challenges of urban warfare, where ISIS's deliberate use of civilians as human shields, staged surrenders to draw fire, and massed vehicle-borne improvised explosive devices (VBIEDs) prolonged fighting and amplified destruction despite coalition precision-guided munitions.^[111]^[87] Launched on 16 October 2016, the operation unfolded in phases: initial shaping efforts isolated Mosul by securing surrounding areas like Qayyarah and Sinjar; eastern Mosul fell by 24 January 2017 after weeks of house-to-house clearing; and western Mosul, including the densely packed Old City, saw the fiercest combat from February to July 2017, with ISIS snipers, drones, and suicide attacks contesting every block. Coalition forces delivered over 1,800 airstrikes in support, targeting ISIS command nodes and VBIED factories while minimizing collateral through joint terminal attack controllers embedded with ISF units, though the group's embedding among 1–1.5 million civilians constrained options. Peshmerga advances from the north fixed ISIS forces, while PMF elements secured the south, preventing major reinforcements or escapes.^[87]^[111] By mid-June, ISF cleared most of west Mosul, collapsing ISIS defenses around the al-Nuri Mosque, where leader Abu Bakr al-Baghdadi had declared the caliphate in 2014; Iraqi Prime Minister Haider al-Abadi declared full liberation on 10 July 2017, though isolated fighting continued into August.^[112] Casualties reflected the attritional nature of the siege. ISF losses included 1,000–1,500 killed and approximately 8,000 wounded, with CTS units suffering up to 60% casualty rates from leading assaults into kill zones; conventional Iraqi forces incurred over 10,000 total casualties. ISIS sustained thousands of fatalities, evidenced by mass graves and battlefield counts, severely degrading its combat capability in Iraq. Civilian deaths ranged from a UN-confirmed minimum of 2,521 to estimates of 9,000–11,000, with about 3,200 linked to coalition airstrikes and ISF indirect fire amid ISIS tactics that funneled noncombatants into combat zones and executed those attempting flight; broader tolls during occupation and battle exceeded 10,000 per household surveys.^[111]^[112]^[87] The victory dismantled ISIS's largest urban stronghold, enabling subsequent collapses of its caliphate in Iraq and pressuring the group to revert to insurgency, but at the cost of 44% of Mosul's pre-battle population of 1.8 million displaced and over 10 million tons of rubble requiring an estimated $2 billion in reconstruction. Coalition non-combat support proved decisive in enabling ISF advances without U.S. ground troops, though the battle underscored limits in isolating modern megacities and the primacy of ground maneuver over airpower alone in dense terrain.^[112]^[87]

Raqqa Campaign

The Raqqa campaign (2016–2017) formed a critical phase of Operation Inherent Resolve, targeting the expulsion of the Islamic State (ISIS) from Raqqa, the Syrian city designated as the group's de facto capital and administrative hub of its proclaimed caliphate. The effort relied on the Syrian Democratic Forces (SDF)—a primarily Kurdish-led alliance incorporating Arab and other local militias—as the primary ground partner, bolstered by the US-led Global Coalition's precision airstrikes, artillery support from forward positions, intelligence sharing, and approximately 900 US special operations personnel providing advice and enabling capabilities. Preparatory operations in late 2016, including the Northern Raqqa offensive, severed ISIS supply lines and isolated the city by liberating surrounding rural areas, setting conditions for the urban assault.^[85]^[113] The decisive urban phase commenced on June 6, 2017, when SDF units advanced into Raqqa from the north, east, and west, encountering fortified ISIS defenses comprising tunnels, sniper positions, booby-trapped buildings, and vehicle-borne improvised explosive devices. ISIS fighters, estimated at 2,500 to 4,000 including foreign contingents, integrated into densely populated neighborhoods, using civilians—whom they prevented from fleeing through threats, mines, and blockades—as shields to complicate coalition targeting and SDF maneuvers. Coalition aircraft conducted thousands of strikes over the four-month battle, dropping more munitions than any US Marine or Army battalion since the Vietnam War, which proved instrumental in degrading ISIS command nodes and fortifications but also contributed to extensive urban destruction, with 60-80% of Raqqa reduced to rubble by the operation's end.^[114]^[115]^[116] Progress was methodical but protracted due to ISIS's attrition tactics and the challenges of house-to-house clearing in a confined urban environment housing up to 300,000 civilians at the outset. SDF forces captured key districts incrementally, including the national hospital in July and the stadium in August, confining ISIS to the city center by September. The campaign concluded on October 17, 2017, when SDF commanders declared Raqqa cleared of organized ISIS resistance, followed by an official announcement of liberation on October 20; this loss compelled ISIS leadership to disperse, eroding its territorial caliphate pretense.^[117]^[85] Casualties reflected the battle's intensity: SDF losses exceeded 900 fighters killed, underscoring the ground partner's commitment despite limited heavy weaponry. ISIS sustained heavy attrition, with coalition estimates indicating thousands of combatants killed through strikes and direct engagements, though exact figures remain unverified due to the group's opaque structure. Civilian deaths, trapped amid crossfire and ISIS obstructions to evacuation, varied sharply by source; the US Department of Defense confirmed around 200-300 from coalition actions in Raqqa via post-strike assessments, emphasizing precautions like warnings and proportional targeting, while non-governmental organizations such as Amnesty International and Airwars documented over 1,600 based on field investigations and witness accounts—discrepancies attributable to differing methodologies, with NGOs potentially inflating via unconfirmed reports and official tallies undercounting indirect effects or secondary explosions from ISIS munitions. ISIS's deliberate embedding in civilian areas causally amplified harm, as evidenced by widespread booby-trapping and refusal to permit exits, rendering the city a deliberate "death trap" despite coalition mitigation efforts.^[113]^[118]

Operation Odyssey Lightning and Others

Operation Odyssey Lightning was a U.S. Africa Command (AFRICOM)-led military campaign conducted from August 1, 2016, to December 19, 2016, to support Libyan forces affiliated with the United Nations-backed Government of National Accord (GNA) in expelling Islamic State (ISIS) militants from the city of Sirte.^[119] ^[120] The operation authorized precision airstrikes, remotely piloted aircraft missions, and naval gunfire support from the 22nd Marine Expeditionary Unit to target ISIS command centers, fighting positions, and logistics in Sirte, where the group had established a de facto provincial capital since early 2015.^[121] ^[122] Over 550 strikes were executed, enabling GNA-aligned militias to advance through urban combat and reclaim key districts.^[123] U.S. forces, operating from naval assets in the Mediterranean and air bases in the region, coordinated with GNA ground elements to degrade ISIS capabilities, including the destruction of vehicle-borne improvised explosive devices and foreign fighter networks.^[124] Specific missions, such as long-range strikes by U.S. Air Force B-52s and A-10s, delivered 85 precision-guided munitions, killing an estimated 100 ISIS combatants in a single coordinated effort on September 20, 2017—though this reflects ongoing assessment rather than total figures.^[124] No U.S. personnel fatalities were reported, and the campaign concluded with Sirte declared liberated, marking a significant territorial setback for ISIS in North Africa outside its core caliphate in Iraq and Syria.^[119] However, ISIS remnants persisted in rural areas of Libya post-operation, conducting sporadic attacks.^[123] Authorized under the 2001 Authorization for Use of Military Force, Odyssey Lightning represented a parallel counter-ISIS effort to Operation Inherent Resolve's focus on Iraq and Syria, though conducted under AFRICOM rather than U.S. Central Command.^[125] Civilian casualty assessments varied: U.S. military reviews emphasized precision targeting to minimize noncombatant harm, with post-strike battle damage assessments guiding operations, but independent monitoring groups reported between 34 and 70 civilian deaths from coalition strikes in Sirte, attributing some to secondary explosions or misidentified targets amid dense urban fighting.^[126] ^[123] Other counter-ISIS sub-operations beyond Iraq and Syria included pre-Odyssey Lightning airstrikes, such as the February 19, 2016, bombing of an ISIS training camp near Sabratha that killed dozens of militants, including a Tunisian bomb-maker linked to attacks in Europe.^[125] AFRICOM also conducted targeted strikes against ISIS affiliates in Somalia, such as the October 3, 2017, operation against ISIS-Somalia leadership, though these were episodic and not formally subsumed under Inherent Resolve.^[119] These efforts collectively aimed to disrupt ISIS's external operations and training networks, prioritizing kinetic action against high-value targets while relying on local partners for sustained ground control.^[127]

Effectiveness and Outcomes

Territorial and Organizational Degradation of ISIS

The territorial caliphate declared by ISIS in June 2014 reached its peak extent in late 2014 to early 2015, controlling approximately 40 percent of Iraq's territory and about one-third of Syria's, totaling over 200,000 square kilometers across urban centers, oil fields, and rural areas that generated revenue through extortion, taxation, and resource extraction.^[13] Operation Inherent Resolve's coalition airstrikes, numbering over 30,000 by 2019, combined with ground offensives by Iraqi Security Forces and Syrian Democratic Forces, systematically eroded this control; key milestones included the recapture of Ramadi in December 2015, Fallujah in June 2016, and major contractions following the 2017 battles for Mosul and Raqqa, where ISIS lost 95 percent of its territory by December 2017.^[14] The final territorial defeat occurred on March 23, 2019, when partner forces cleared the last ISIS-held pocket in Baghouz, Syria, eliminating the group's ability to govern populated areas or project state-like functions. Organizationally, ISIS's hierarchical structure and operational networks were degraded through targeted killings of mid- and high-level commanders, with coalition operations eliminating at least 180 senior leaders by mid-2017 alone, disrupting command-and-control, logistics, and foreign fighter recruitment pipelines that had peaked at 30,000-40,000 inflows.^[128] Financial backbone shattered as airstrikes neutralized over 90 percent of ISIS's oil production capacity by 2016 and severed extortion rackets in liberated cities, slashing annual revenues from an estimated $1-2 billion at peak to under $50 million by 2019.^[39] Post-2019, the group devolved into decentralized insurgent cells, with capabilities confined to survival-mode guerrilla tactics; as of 2023-2024, ongoing coalition and partner strikes continued to inflict leadership losses and prevent reconstitution, maintaining ISIS at 5,000-10,000 fighters regionally while hindering large-scale attacks.^[129] This degradation stemmed from persistent pressure rather than decisive eradication, as ISIS exploited ungoverned spaces and sectarian grievances to sustain low-level operations despite territorial collapse.^[130]

Leadership Eliminations and Counter-Network Efforts

Combined Joint Task Force - Operation Inherent Resolve (CJTF-OIR) prioritized the elimination of ISIS leadership through precision strikes, special operations raids, and intelligence-enabled targeting to fracture the group's command-and-control apparatus and reduce its ability to coordinate attacks. These efforts, often conducted in partnership with Iraqi Security Forces (ISF) and Syrian Democratic Forces (SDF), resulted in the confirmed deaths of numerous senior figures, including provincial emirs, military commanders, and financiers. By 2017, coalition airstrikes had killed several high-ranking ISIS officials, such as Abu Ahmad al-Muhajir, a senior military officer responsible for procurement and research & development, on September 7, 2017, in Raqqa, Syria.^[131] Additional strikes that year eliminated other leaders involved in external operations and oil smuggling networks. A pivotal achievement occurred on October 26, 2019, when U.S. Delta Force operators raided a compound in Barisha, Idlib province, Syria, killing ISIS caliph Abu Bakr al-Baghdadi, who detonated a suicide vest during the assault, alongside several aides and family members.^[132] The operation, supported by CJTF-OIR intelligence and unmanned aerial surveillance, prevented the escape of key targets and yielded valuable digital media for further network disruption. Subsequent leadership losses included Abu Ibrahim al-Hashimi al-Qurayshi in a 2022 U.S. raid and ongoing eliminations, such as a senior ISIS official in Atmeh, Syria, on August 24, 2025, via U.S. special operations.^[133] In October 2024 alone, ISF, enabled by CJTF-OIR, conducted strikes and raids killing multiple senior ISIS members in Iraq's Anbar province and elsewhere.^[134] These actions contributed to a pattern of repeated leadership turnover, with ISIS replacing fallen commanders but facing persistent operational challenges due to degraded hierarchies.^[135] Beyond direct eliminations, counter-network efforts targeted ISIS's support infrastructure, including financial facilitators, logistics enablers, and propaganda operatives, to erode the group's resilience and recruitment. Coalition strikes frequently hit financiers, such as a key ISIS oil smuggler killed in Abu Kamal, Syria, on June 16, 2017.^[136] The U.S. Treasury Department, in coordination with CJTF-OIR partners, designated and disrupted ISIS funding channels, including sanctions on entities handling extortion, extortion rackets, and cryptocurrency flows that sustained remnant cells.^[137] Army Special Operations Forces played a central role in these operations, conducting raids to capture or kill mid-level facilitators and seize documents revealing external attack plots and foreign fighter pipelines.^[40] The Counter ISIS Finance Group, co-led by the U.S., Italy, and Saudi Arabia, coordinated global measures to interdict donations and hawala networks, issuing joint statements in 2024 emphasizing comprehensive strategies against ISIS financing.^[138] These multifaceted disruptions, combined with partner capacity-building, aimed to prevent ISIS regeneration by severing resource flows and intelligence networks, though the group's decentralized structure allowed some adaptation through local cells.^[139]

Long-Term Counter-Terrorism Impacts

Coalition and partner force operations under Operation Inherent Resolve have significantly degraded ISIS leadership structures and operational networks in Iraq and Syria, transitioning the group from territorial control to a persistent but diminished insurgency incapable of mounting large-scale attacks in Iraq as of 2025.^[140]^[4] This degradation stems from sustained strikes, intelligence-driven raids, and partner-led ground operations that eliminated thousands of fighters and mid-level commanders, disrupting command-and-control and financing channels essential for coordinated offensives.^[141] Monthly ISIS attack data from 2023 to mid-2025 indicates low and stable levels of activity, with no notable uptick in sophistication or frequency, reflecting the enduring pressure from Iraqi Counter-Terrorism Service units trained and advised by coalition forces.^[1] Capacity-building efforts have equipped local partners, including Iraqi security forces and Syrian Democratic Forces, with enhanced intelligence, logistics, and tactical capabilities, enabling independent defeat of ISIS remnants and reducing reliance on direct U.S. intervention for routine operations.^[4] These initiatives, part of a broader U.S. strategy integrating military advising with governance support, have sustained partner-led raids that prevented ISIS from regaining momentum, as seen in the lack of significant operational improvements in Syria through 2025.^[30] However, systemic governance failures in Iraq and Syria—such as corruption, sectarian divisions, and weak state control in Sunni-majority areas—persist as causal enablers for low-level ISIS recruitment and safe havens, underscoring that military degradation alone does not eradicate ideological resilience or address root vulnerabilities.^[142] Globally, OIR's disruption of ISIS's core has indirectly constrained resource flows to affiliates, but the group's decentralized model has allowed branches in Africa (e.g., Islamic State West Africa Province) and Afghanistan (Islamic State Khorasan Province) to expand operations post-2019 territorial losses, conducting high-profile attacks like the 2021 Kabul airport bombing.^[143]^[144] While core-area successes limited foreign fighter pipelines and propaganda amplification from a caliphate, affiliates have adapted by exploiting local grievances, resulting in rising attack volumes in the Sahel and South Asia from 2020 to 2025, independent of direct OIR involvement.^[3] Enduring counter-terrorism requires ongoing intelligence sharing and targeted strikes beyond Iraq and Syria to mitigate this diffusion, as ISIS's transnational ideology continues to inspire lone-actor plots worldwide despite the coalition's localized victories.^[143]

Casualties and Humanitarian Considerations

Coalition and Partner Force Losses

United States military personnel have incurred the bulk of coalition fatalities, with 118 total deaths recorded as of October 20, 2025, comprising 23 hostile deaths and 95 non-hostile deaths from causes such as accidents, training incidents, and medical issues.^[145]^[146] These losses occurred primarily during advisory roles supporting partner ground operations, with hostile incidents often involving indirect fire from ISIS rockets or improvised explosive devices. Wounded-in-action figures for U.S. forces exceed 400, reflecting sporadic engagements rather than sustained infantry combat.^[147] Non-U.S. coalition partners, operating in similar support capacities, reported minimal fatalities, totaling under 50 across participating nations including the United Kingdom (at least one combat death in 2016), France, Australia, and Jordan.^[148] The low coalition toll underscores the operation's reliance on precision airstrikes—over 34,000 conducted by 2020—and special operations raids, which minimized direct exposure to ISIS defenses.^[149] Partner forces conducting frontline assaults absorbed the preponderance of casualties, with no centralized coalition tally available due to varying reporting standards among Iraqi, Kurdish, and Syrian groups. Iraqi Security Forces endured heavy attrition in urban battles like Mosul (2016–2017), where hundreds to over 1,000 personnel were killed amid house-to-house fighting against entrenched ISIS positions, though exact aggregates remain unverified beyond partial government disclosures. Kurdish Peshmerga units lost hundreds in northern Iraq offensives, such as the push to Kirkuk in 2014 and subsequent ISIS counterattacks. The Syrian Democratic Forces claimed over 11,000 fighters killed since inception in battles including Raqqa (2017) and the Baghouz pocket (2019), reflecting intense close-quarters combat in eastern Syria.^[150] These disparities highlight how coalition air and intelligence support enabled partners to degrade ISIS territorially while shifting ground risks to local troops.

Adversary Casualties

The U.S.-led coalition under Operation Inherent Resolve inflicted substantial casualties on the Islamic State (ISIS) and affiliated jihadist groups through airstrikes, advisory support to ground partners, and special operations raids. Pentagon assessments estimated that coalition efforts contributed to the deaths of approximately 80,000 ISIS fighters in Iraq and Syria from 2014 to 2019, when the group's territorial caliphate was dismantled.^[151] These figures encompassed battle damage assessments from airstrikes, partner force reports of enemy killed in action (EKIA), and intelligence on leadership eliminations, though independent verification remains challenging due to urban combat environments, booby-trapped sites, and ISIS tactics of blending with civilians.^[93] Ground partners, including Iraqi Security Forces, Peshmerga, and Syrian Democratic Forces, bore the brunt of close-quarters fighting and reported thousands of additional ISIS casualties in major engagements, often enabled by coalition close air support exceeding 30,000 strikes by 2019.^[152] For instance, in Iraq, Counter-Terrorism Service operations in 2021 alone yielded 47 EKIA from nine coalition-supported strikes. Strikes also targeted non-ISIS adversaries, such as al-Qaeda's Khorasan Group in Syria, with hundreds killed in early phases to disrupt external plotting networks.^[153] Post-2019, as OIR transitioned to advisory and counterterrorism missions, adversary losses continued at a reduced rate amid ISIS insurgent remnants. CENTCOM reported operations in 2024 killing dozens in joint raids, such as 15 ISIS fighters in an August raid with Iraqi forces near Kirkuk.^[154] By mid-2025, ongoing partner-led actions enabled by coalition intelligence had further degraded ISIS networks, though exact cumulative EKIA beyond initial estimates are not publicly aggregated due to classification and focus on operational impacts over raw counts.^[108] Analysts note potential overestimation in early figures from optimistic battle reports, balanced against ISIS's documented attrition from an estimated peak of 30,000 core fighters to scattered cells.^[155]

Civilian Casualties and Mitigation Efforts

The U.S.-led coalition in Operation Inherent Resolve has acknowledged at least 1,437 unintentional civilian deaths from its operations since August 2014, based on assessments of credible reports through March 2022, with subsequent quarterly reviews confirming additional incidents but maintaining low annual totals in later years. Independent monitoring by Airwars has documented over 3,000 alleged incidents of civilian harm from coalition actions in Iraq and Syria, with estimates of 8,000 to 13,000 civilian deaths attributed to airstrikes, though many remain unverified and include claims from local sources potentially influenced by ISIS propaganda or incomplete data.^[156] The discrepancy arises from the coalition's rigorous post-incident investigations, which often classify reports as non-credible after reviewing intelligence, video evidence, and partner inputs, while NGOs like Airwars rely on media and eyewitness accounts that may conflate coalition strikes with ISIS actions or ground fighting. Peak casualties occurred during urban battles against ISIS strongholds, such as Mosul in 2016–2017 and Raqqa in 2017, where coalition airstrikes supported ground advances amid ISIS's deliberate use of human shields and embedding in civilian infrastructure, contributing to higher collateral damage despite efforts to target militants.^[126] The U.S. Department of Defense's annual reports note approximately 120 civilian deaths in 2018 and fewer in subsequent years as territorial fighting subsided, with 2022 seeing only 14 potential incident reports received, most deemed non-credible after review.^[157] By early 2025, quarterly assessments, such as for January–March, attributed isolated deaths to partner ground forces rather than coalition strikes, reflecting reduced operational tempo. Mitigation efforts by CJTF-OIR emphasize precision-guided munitions, multi-intelligence targeting to distinguish combatants, and strict rules of engagement requiring high confidence in minimal civilian presence before strikes.^[158] Pre-strike protocols include thorough target reviews, collaboration with local partners for on-ground verification, and real-time monitoring to abort if civilian risks emerge, as demonstrated in investigations of specific incidents like the June 2017 Raqqa strike where three civilian deaths were confirmed and led to procedural refinements.^[159] Post-incident, the coalition conducts rapid credibility assessments and full investigations for credible claims, sharing findings publicly to build trust, while training partners on civilian harm avoidance and providing condolence payments in verified cases. These measures, prioritized since 2014, aim to balance operational necessity against risks in dense, contested environments where ISIS tactics—such as forcing civilians into combat zones—causally amplified unintended losses.^[160]

Controversies and Criticisms

Allegations of Excessive Force and War Crimes

Amnesty International and Airwars documented over 1,600 civilian deaths from US-led coalition airstrikes during the 2017 battle for Raqqa, Syria, alleging that at least 14 strikes may have been indiscriminate or disproportionate, potentially constituting war crimes under international humanitarian law due to failures to distinguish between combatants and civilians despite available intelligence.^[118]^[161] The report, based on field investigations of 42 strike sites and interviews with 112 survivors, highlighted cases where entire families were killed in strikes on residential areas, amid ISIS's tactic of embedding fighters among civilians and using human shields, which increased collateral risks but did not absolve alleged violations per the NGOs.^[118] The coalition contested the figures, maintaining that strikes adhered to proportionality principles and that higher estimates often included ISIS-perpetrated deaths or unverified claims.^[162] In the 2016-2017 battle for Mosul, Iraq, a New York Times investigation identified multiple coalition airstrikes that killed hundreds of civilians, including a March 17, 2017, strike on a building packed with families, resulting in up to 200 deaths despite prior warnings to Iraqi forces about civilian presence.^[163]^[164] Leaked Pentagon assessments revealed patterns of flawed targeting intelligence and rushed decisions in dense urban fighting, where ISIS held civilians hostage in booby-trapped structures, complicating strike assessments.^[165] The US acknowledged some incidents, attributing them to erroneous battle damage assessments or ISIS deception, and conducted investigations under rules of engagement emphasizing civilian protection, but critics argued these often dismissed allegations without full accountability.^[165]^[166] A March 18, 2019, coalition airstrike near Baghouz, Syria, targeted an ISIS compound but killed an estimated 60-70 civilians, including women and children, who were sheltering amid fleeing fighters; a US Central Command review in 2021 deemed the strike lawful, citing active combat and proportional force against valid military targets, though it noted the unintended civilian toll in a chaotic endgame scenario.^[167] Airwars and media reports estimated higher casualties based on witness accounts, prompting calls for independent probes, but no criminal findings emerged.^[167] Broader allegations of excessive force center on the coalition's estimated 30,000+ airstrikes since 2014, with NGOs like Human Rights Watch and Airwars claiming thousands of civilian deaths—far exceeding the coalition's official tally of about 1,400 unintentional fatalities—due to inadequate precautions in ISIS-held urban areas.^[156]^[168] No coalition personnel have been prosecuted for war crimes related to OIR, as US investigations typically conclude operations complied with law of armed conflict standards, emphasizing ISIS's deliberate civilian endangerment; however, legal scholars argue coalitions bear heightened investigative duties for potential violations.^[8]^[169] These claims persist amid debates over transparency, with DoD retaining records of allegations but lacking a comprehensive tracking system for patterns.^[169]

Geopolitical Trade-Offs and Proxy Dynamics

The U.S.-led coalition's emphasis on degrading ISIS territories often necessitated alliances with local proxies that advanced competing geopolitical agendas, creating strategic trade-offs between short-term tactical gains and long-term regional stability. In Iraq, coalition support for the Popular Mobilization Forces (PMF)—Shia militias backed by Iran—enabled rapid advances against ISIS, such as the liberation of Mosul in July 2017, but entrenched Iranian influence by formalizing PMF integration into Iraq's security apparatus under the 2016 Popular Mobilization Commission law.^[104] This dynamic fueled narratives of proxy competition, where U.S. tolerance of Iran-aligned groups to expedite ISIS defeat inadvertently bolstered Tehran's "land bridge" to Syria, complicating post-2017 efforts to curb militia autonomy despite U.S. designations of groups like Kata'ib Hezbollah as terrorists.^[170] In Syria, the coalition's partnership with the Kurdish-led Syrian Democratic Forces (SDF), dominated by the People's Protection Units (YPG), proved decisive in expelling ISIS from Raqqa in October 2017 and defeating its caliphate remnant at Baghouz in March 2019, with U.S. provision of over $5 billion in aid and training from 2015 onward.^[171] However, Turkey's designation of the YPG as an extension of the Kurdistan Workers' Party (PKK)—a terrorist group per NATO consensus—prompted Ankara's cross-border operations, including Operation Olive Branch in 2018, which disrupted SDF cohesion and exposed U.S. vulnerabilities in balancing anti-ISIS imperatives against alliance frictions with a key NATO partner.^[172] These tensions peaked in October 2019 when U.S. troop withdrawals facilitated Turkish incursions, allowing ISIS sleeper cells to exploit SDF diversions and highlighting the trade-off of empowering a non-state actor with separatist aspirations over sustained Turkish cooperation.^[173] Russian intervention in Syria from September 2015, ostensibly against ISIS but primarily targeting U.S.-backed rebels, preserved Bashar al-Assad's regime by recapturing Aleppo in December 2016 and enabling Syrian Arab Army advances that sidelined coalition efforts beyond ISIS-specific zones.^[174] Moscow's airstrikes, which hit non-ISIS targets in 80-90% of initial sorties per independent monitoring, allowed Assad to consolidate control over 60% of Syrian territory by 2018, shifting the conflict's proxy balance toward Iran-Russia axes at the expense of Western goals for a post-Assad transition.^[175] This de facto partitioning—where coalition operations confined to eastern Syria ceded western theaters—underscored a core trade-off: ISIS territorial defeat without broader regime pressure enabled adversarial proxies to entrench, fostering persistent Iranian entrenchment and Russian naval basing at Tartus, with implications enduring into 2025 amid Assad's ouster risks.^[176] Overall, proxy reliance in Operation Inherent Resolve amplified multipolar rivalries, as U.S. enablement of SDF and PMF forces yielded ISIS losses—over 100,000 fighters killed by 2020—but empowered Iran to expand its militia network to 200,000 personnel across Iraq and Syria, strained NATO unity, and permitted Russian-Assad consolidation without direct confrontation.^[177] These dynamics revealed causal trade-offs in counterinsurgency, where localized proxy efficacy against a singular threat like ISIS often deferred accountability for state fragility and hegemonic competition.^[178]

Strategic and Domestic Debates

The "by, with, and through" strategy employed in Operation Inherent Resolve, which emphasized advising and enabling local partner forces rather than large-scale U.S. ground troop deployments, succeeded in degrading ISIS's territorial caliphate by 2019 but sparked debate over its scalability and risks in future conflicts. Proponents, including U.S. military analysts, argued that this approach minimized American casualties—totaling fewer than 100 U.S. combat deaths from 2014 to 2025—while building indigenous capacity, as evidenced by Iraqi Security Forces reclaiming Mosul in 2017 with U.S. advisory support.^[19] Critics, however, contended that inconsistent doctrine and heavy reliance on partners like the Iraqi army, which collapsed in 2014, prolonged the campaign and allowed ISIS to adapt into a resilient insurgency, with over 10,000 estimated ISIS fighters remaining active in Iraq and Syria as of 2024.^[19] ^[179] Airpower's dominance in OIR, involving over 134,000 coalition strikes from 2014 to 2019, was hailed for enabling ground advances without committing U.S. divisions, yet strategic analyses highlighted limitations such as rigid 72-hour air-tasking cycles that hindered rapid response to ISIS's mobile tactics, potentially extending operations by months.^[179] This approach also raised concerns about geopolitical trade-offs, including bolstering Iran-backed Shia militias in Iraq, which gained influence post-ISIS territorial defeat, and arming Syrian Democratic Forces dominated by Kurdish YPG, straining U.S.-Turkey relations due to Ankara's designation of the YPG as a PKK extension.^[180] Debates intensified over enduring U.S. presence, with some experts warning that full withdrawal risks ISIS resurgence, as seen in minor territorial gains by the group in 2023-2024, while others viewed indefinite basing as a vulnerability to militia attacks, exemplified by over 150 incidents targeting U.S. forces in Iraq from 2021 to 2024.^[104] Domestically, Operation Inherent Resolve fueled partisan divides, with Republicans criticizing the Obama administration's 2014 initiation for lacking a comprehensive strategy and ground combat troops, arguing it echoed failed light-footprint efforts in Afghanistan and enabled ISIS's initial expansion from Al-Qaeda in Iraq remnants.^[180] The Trump administration's partial Syria troop drawdown in 2019, reducing U.S. forces from 2,000 to about 900, drew bipartisan backlash for risking ISIS prison breaks, though it aligned with vows to end "endless wars," amid reports of over $60 billion in U.S. costs by 2020.^[181] Congressional debates centered on authorization, as presidents from Obama onward relied on the 2001 Authorization for Use of Military Force without new legislation specific to ISIS, prompting legal scholars and lawmakers to question its constitutionality for operations against a non-Al-Qaeda entity, leading to repeated but unsuccessful calls for repeal or replacement by 2023.^[182] By 2025, as the coalition announced the mission's transition in Iraq—shifting to bilateral advisory roles—the Biden administration faced scrutiny over sustaining minimal footprints (around 2,500 troops) amid domestic war fatigue, with polls showing 60% of Americans opposing prolonged Middle East engagements in 2024 surveys.^[9] Advocates for drawdown cited fiscal burdens and low direct threats to U.S. homeland security post-caliphate, while opponents, including defense hawks, emphasized persistent ISIS attack planning capabilities, as disrupted in 2024 coalition raids killing key leaders.^[17] These tensions underscored broader realism in U.S. policy, prioritizing deterrence of great-power rivals over indefinite counterinsurgency.

Transition and Legacy

2025 Winding Down in Iraq and Syria

In September 2025, the Global Coalition to Defeat ISIS formally ended its combat mission in Iraq, transitioning Combined Joint Task Force-Operation Inherent Resolve (CJTF-OIR) activities to a bilateral U.S.-Iraq security partnership framework focused on advisory support and capacity building for Iraqi forces.^[9] This followed a two-phase plan agreed in September 2024, with Phase I involving the reposturing of coalition forces from multiple Iraqi bases to fewer advisory sites, completed by mid-2025, and Phase II emphasizing the handover of counter-ISIS operations to Iraqi security forces.^[149]^[183] U.S. troop levels in Iraq, which stood at approximately 2,500 at the start of 2025, began further reductions in October 2025, with forces consolidating in Erbil and other key locations to minimize exposure while maintaining targeted intelligence and logistics support against ISIS remnants.^[184]^[105] The drawdown aligned with Iraqi government demands to end foreign combat presence, amid ongoing militia attacks on U.S. positions that had prompted retaliatory strikes earlier in the year, though overall coalition strikes decreased as local forces assumed lead roles.^[185] In Syria, CJTF-OIR operations persisted without a fixed end date, but U.S. forces underwent parallel reductions, dropping from over 2,000 personnel at the year's outset to between 900 and 1,000 by October 2025, including the closure or handover of multiple small bases in northeastern Syria to partner Kurdish-led Syrian Democratic Forces (SDF).^[186]^[187] These adjustments reflected a strategic shift toward enabling SDF-led counterterrorism, with U.S. aviation and special operations providing overwatch against ISIS cells, even as the fall of the Assad regime in late 2024 introduced new instability risks and prompted accelerated base consolidations.^[104] Despite the drawdown, coalition reports noted sustained ISIS pressure, with partner forces detaining thousands of fighters and disrupting attack networks through mid-2025.^[107] The winding down process across both countries emphasized long-term stability over indefinite presence, with U.S. officials citing degraded ISIS capabilities—territory reduced to rural pockets—as justification for the transition, though analysts warned of resurgence risks without vigilant partner support.^[188] Foreign assistance programs, including those from USAID, were paused or scaled back in contested areas of Iraq and Syria during Q2 2025, prioritizing stabilization aid to prevent governance vacuums that could enable extremist regrouping.^[30] By late 2025, CJTF-OIR's role had evolved into a smaller, enduring advisory mission, retaining capabilities for rapid response while ceding operational primacy to regional allies.^[4]

Persistent Threats and Resurgence Risks

Despite the territorial defeat of the Islamic State of Iraq and Syria (ISIS) caliphate in March 2019, the group sustains persistent low-level insurgent operations in Iraq and Syria, conducting guerrilla-style attacks on security forces and infrastructure.^[189] In 2025, Combined Joint Task Force-Operation Inherent Resolve (CJTF-OIR) partners reported ongoing ISIS threats to detention facilities holding approximately 10,000 fighters and displaced persons camps in northeastern Syria, where the group has attempted infiltrations and assaults to free detainees.^[30] For instance, Syrian Democratic Forces, supported by CENTCOM, neutralized ISIS elements in operations near Dayr az-Zawr in early January 2025, underscoring the group's operational tempo.^[108] Key enablers of this persistence include ungoverned spaces, porous borders, and local grievances such as corruption and economic stagnation, which facilitate recruitment and logistics for ISIS cells estimated at several thousand active fighters in the region.^[1] U.S. Central Command strikes in July and September 2025 targeted senior ISIS figures in Al-Bab and other Syrian areas, eliminating operatives planning external attacks and posing direct threats to coalition partners.^[190]^[191] These actions reflect ISIS's decentralized structure, which relies on sleeper cells and propaganda to inspire lone-actor plots globally while maintaining a core threat in its origin theaters.^[192] Resurgence risks escalate with the planned CJTF-OIR transition and U.S. troop reductions in Iraq and Syria by late 2025, potentially straining partner forces like the Iraqi Security Forces and SDF amid governance vacuums.^[9] The fall of the Assad regime in Syria has created exploitable chaos, allowing ISIS to seek reconstitution of attack capabilities through released prisoners and tribal alliances, as assessed by U.S. intelligence.^[193] Weak state control in rural areas, coupled with ISIS affiliates' external funding networks, heightens the probability of territorial gains if counter-ISIS pressure diminishes, threatening U.S. interests in regional stability.^[194] Sustained partner capacity-building remains critical to mitigate these dynamics, though historical patterns of insurgent adaptation post-major defeats indicate enduring vulnerability without vigilant enforcement.^[195]

Broader Lessons for Counter-Insurgency

Operation Inherent Resolve demonstrated the viability of a "by, with, and through" strategy in counter-insurgency, wherein U.S. and coalition forces enabled local partners such as the Iraqi Security Forces and Syrian Democratic Forces to lead ground operations against ISIS, thereby avoiding large-scale direct interventions that characterized earlier campaigns like Operation Iraqi Freedom. This approach facilitated the territorial defeat of ISIS's self-proclaimed caliphate by March 2019, including the liberation of approximately 35,000 square miles in Syria and 70 cities in Iraq, while limiting U.S. fatalities to fewer than 100 from 2014 to 2020.^[40]^[19] However, success hinged on motivated indigenous partners and sustained coalition enablers, underscoring that such models are not universally replicable without pre-existing partner capacity and political alignment.^[19] The integration of airpower, special operations, and limited ground enablers proved essential for amplifying partner effectiveness, with coalition aircraft conducting strikes that provided decisive close air support during major offensives like the battles for Mosul and Raqqa. U.S. Army Special Operations Forces played a pivotal role in advising, training, and conducting targeted raids, adapting tactics to support irregular allies and contributing to the destruction of ISIS leadership and infrastructure.^[40]^[5] Ground-based contributions, including embedded advisors coordinating surface fires and intelligence, enhanced leverage in urban fights such as Ramadi and Fallujah, where U.S. kinetic effects compensated for partner shortcomings without escalating to full combat deployments.^[19] These elements highlighted that counter-insurgency requires synchronized joint capabilities, as airpower alone could not achieve territorial control, emphasizing the need for ground partners to hold gains.^[5] Interagency coordination emerged as a critical but challenging factor, with shared facilities like Forward Operating Base Union III enabling military-diplomatic synergy for logistics and stabilization in Iraq, yet divergent risk tolerances—such as the State Department's partial evacuations amid militia threats—disrupted sustainment efforts.^[196] Effective counter-insurgency thus demands integrated planning across military, diplomatic, and developmental lines to transition from kinetic operations to governance support, preventing vacuums that insurgents exploit.^[196] Broader implications include the recognition that territorial victories yield only temporary gains without addressing underlying state weaknesses, as evidenced by ISIS's persistence as a low-level insurgency post-2019 through guerrilla tactics and external financing.^[40] Future operations should prioritize doctrinal formalization of partner-enabling tactics, investment in junior leader training for adaptive advising, and realistic assessments of partner reliability to mitigate resurgence risks, avoiding over-optimism about light-footprint models in contexts lacking viable locals.^[19]