The protracted campaigns to liberate Mosul and key Syrian territories from the Islamic State of Iraq and Syria (ISIS) represent critical turning points in the global fight against this extremist organization. From early 2017 onwards, a series of strategic defeats, marked by substantial territorial loss and the decimation of its fighting force, fundamentally altered ISIS's capacity to wage conventional warfare and maintain its self-proclaimed caliphate. The fall of Mosul, which served as the de facto capital of ISIS's operations in Iraq, and the subsequent erosion of its territorial hold across northern and eastern Syria, were not merely symbolic victories; they were the direct result of concerted military efforts that inflicted profound and irreparable damage on the group’s infrastructure, leadership, and operational capabilities.
The battle for Mosul, commencing in October 2016 and concluding with the city's full liberation in July 2017, was a brutal and devastating affair. The Iraqi Security Forces, supported by the United States-led coalition’s airpower and advisors, engaged in some of the most intense urban warfare seen in decades. ISIS fighters, estimated to number between 5,000 and 10,000 at the outset of the offensive, had fortified the city extensively. They employed tactics such as tunnels, suicide bombings, and the use of human shields, turning the densely populated western districts into a death trap. The loss of Mosul was significant for ISIS on multiple fronts. Militarily, it meant the forfeiture of a major urban center that provided a significant recruitment base, a logistical hub, and a symbol of their governance. Economically, it deprived them of revenue generated from taxation, smuggling, and control over key infrastructure. Psychologically, the defeat in their proclaimed capital shattered the illusion of invincibility and the permanence of the caliphate. Reports from the period indicated heavy casualties for ISIS, with thousands killed and captured. The Iraqi Ministry of Defence, for instance, reported the deaths of over 1,000 ISIS fighters in the final weeks of the battle alone.
Concurrently, in Syria, ISIS faced a multi-pronged assault that systematically dismantled its control over swathes of territory. The Syrian Democratic Forces (SDF), a coalition of Kurdish and Arab militias backed by the U.S., spearheaded offensives in northern and eastern Syria. Key territorial losses included the city of Raqqa, ISIS's nominal capital, which fell to the SDF in October 2017 after a four-month siege. The capture of Raqqa was a monumental blow, removing the administrative heart of the caliphate and dealing a severe blow to ISIS's prestige. Beyond Raqqa, the SDF gradually pushed ISIS out of its strongholds along the Euphrates River valley, including towns like Deir ez-Zor and the final holdout of Baghouz Fawqani, which fell in March 2019. These campaigns were characterized by fierce resistance from ISIS, but the overwhelming air superiority of the coalition and the coordinated ground efforts of the SDF proved decisive. The loss of these strategic locations not only denied ISIS territory but also cut off vital supply routes and access to oil fields that had been a significant source of funding. Estimates suggest that by the time of Baghouz’s fall, ISIS had lost over 95% of the territory it once controlled at its peak in 2014.
Beyond the immediate territorial losses, these campaigns had profound implications for ISIS's leadership and operational capacity. Many high-ranking commanders and foreign fighters were killed or captured during the battles for Mosul and the Syrian strongholds. The death of key figures, such as Abu Bakr al-Baghdadi, who was killed in a U.S. raid in October 2019, though occurring after the territorial collapse, was a direct consequence of the pressure exerted during these campaigns that forced the leadership into hiding. The fragmentation of ISIS's command structure and the loss of experienced fighters significantly hampered its ability to mount coordinated attacks. While ISIS has since transitioned into an insurgent and terrorist organization, its ability to project power and control territory was decisively curtailed by the military defeats suffered in Mosul and Syria. The loss of these key territories marked the end of the caliphate as a territorial entity and fundamentally reshaped the nature of the global counter-terrorism effort against the group.