By Eric Vandenbroeck and co-workers

The Return of ISIS

Nine months after the longtime dictator Bashar al-Assad was toppled by a rebel offensive, Syria faces a litany of new challenges. The country, which is now being led by the militant group Hayat Tahrir al-Sham (HTS), is contending with recurring violent sectarian clashes, successive Israeli strikes into Syrian territory, and internal disputes within the new government. Adding to the tumult is a resurgence of one of Syria’s most enduring challenges: the Islamic State, also known as ISIS.

Since the Assad regime was toppled in 2024, ISIS has waged a terror campaign throughout Syria, targeting the new Syrian government, as well as Christian, Shiite, and Kurdish minorities. At its apogee, in 2014, ISIS held roughly a third of the country. Although the group no longer controls any territory in Syria, and its numbers have dwindled from roughly 100,000 fighters to 2,500 fighters today, the group is taking advantage of Syria’s post-Assad chaos to rebuild and reconstitute, presenting fresh obstacles to the country’s long-sought stability.

The group’s targeting capabilities are proving more frequent, precise, and sophisticated than ever, targeting sites well beyond ISIS’s traditional spheres of operation. In June, for example, a suicide bomber linked to ISIS attacked a Greek Orthodox church in Damascus, killing 25 and injuring 63. Two months later, the group launched more than two dozen attacks across northeastern Syria, relying on a combination of guerrilla tactics, including small arms fire, ambushes, assassinations, and improvised explosive devices targeting military checkpoints and government vehicles. Last year, ISIS took responsibility for 294 attacks in Syria, up from 121 in 2023; estimates by the United Nations and human rights groups are even higher.

Such attacks pose a blatant challenge to the new administration’s attempts to stabilize the country. Syria’s already fragile security situation is characterized by frequent clashes between Sunni, Alawite, and Druze communities. As the frequency of terrorist attacks increases, the new Syrian government risks squandering its political legitimacy by failing to protect the country’s minorities. The Syrian population, meanwhile, faces a real possibility of a large-scale terrorist resurgence.

This situation will grow more dangerous still if the Trump administration follows through on its plan, announced in April, to begin withdrawing the approximately 2,000 U.S. troops stationed in the country. Since 2014, the United States has been the linchpin of the Global Coalition to Defeat ISIS, an international counterterrorism group that has worked with local security forces to degrade ISIS. Without Washington’s active involvement, the coalition’s remaining 88 members will struggle to keep ISIS at bay or to adequately support local actors such as the Syrian Democratic Forces (SDF), a mostly Kurdish militia that had recently reached an agreement to merge with the state.

As U.S. troop numbers dwindle and sectarian tensions mount, Syrian forces may find it increasingly difficult to maintain authority in the country. The combined manpower of the Syrian army and the SDF is likely insufficient to prevent an ISIS resurgence. To deter ISIS, therefore, the United States must retain a presence in Syria beyond 2026. A contingent of roughly several hundred U.S. troops will be necessary to support Syria’s new security forces with intelligence, surveillance, reconnaissance, and capacity building. If Washington discontinues counterterrorism support to Damascus, the Syrian government may fail to unify the country and prevent ISIS’s return. If ISIS can regain momentum in Syria, it will inevitably turn to conducting attacks throughout the region and beyond. It behooves Washington to prevent ISIS’s ability to rebuild and once again destabilize the entire Levant.

 

Back With a Vengeance

ISIS sees an opportunity in a divided Syria. Its leaders seek to exploit the country’s sectarian and ideological divisions to recruit new fighters and rebuild their caliphate. Through its attacks, ISIS hopes to demonstrate that the current Syrian government is unable or unwilling to protect the population, especially minorities. A recent report from the UN Monitoring Team concluded that ISIS “will continue to project an external threat . . . if divisions in the country allow a permissive space from where they can plan and execute attacks.”

Indeed, ISIS was especially busy last spring. In March, in the aftermath of violence between Sunni and Alawite communities in the port city of Latakia, ISIS conducted a string of attacks against the SDF. Then in May, the group launched another barrage of attacks following clashes south of Damascus, between the new administration’s security forces and Sunni and Druze communities. That same month, just a week after U.S. President Donald Trump met in Riyadh with Syria’s new president, the former jihadist Ahmed al-Shara, ISIS launched a series of bombings and ambushes across Syria and Iraq. The group also took credit for planting an explosive device that struck a “vehicle of the apostate regime” in southern Syria, resulting in at least seven Syrian army casualties and marking the first recorded attack on Syrian government forces since Assad’s fall. A week later, another bomb attack conducted by ISIS in southern Syria targeted fighters from the Free Syrian Army, a U.S.-backed militia. Notably, all these attacks transpired deep in government-held territory.

This surge in violence has been accompanied by a surge in ISIS propaganda undermining Shara and HTS. In its weekly online newsletter, which reaches and inspires jihadists across the globe, the group routinely maligns Shara and the new government and calls for soldiers in the Syrian military to defect. ISIS propagandists call Shara “Jewlani,” in reference to a conspiracy that the Syrian president, formerly known as Abu Mohammad al-Jolani, is an Israeli Mossad agent installed to initiate regime change inside Syria in favor of Israel. The group has also labeled HTS as kuffar (Arabic for “nonbelievers”) and accused it of being a puppet of the United States and Israel. Since HTS and Shara came to power, Israel has repeatedly carried out strikes and ground incursions into Syria, sowing the very chaos on which ISIS strives.

Alarmingly, ISIS’s growing confidence has coincided with an incremental withdrawal of U.S. forces in Syria and Iraq, placing the future of the Global Coalition into question. In September 2024, the United States and Iraq jointly announced that the Global Coalition would end its military mission in Iraq by the end of September 2025 and in Syria by September 2026. In April, U.S. Central Command announced cuts to its U.S. troop presence in Syria from 2,000 to 1,400 by the end of this year. The United States has already transferred control of three of its eight bases in the country to the SDF, with plans to consolidate to just one base. Although Pentagon commanders have urged the administration to maintain at least a contingent of 500 troops in Syria, the administration has signaled that further cuts may be made. In neighboring Iraq, a parallel process is underway. With a withdrawal goal set for the end of 2026, U.S. forces are reconsolidating from Baghdad to Erbil, in Iraqi Kurdistan, and plan to drastically reduce their presence to a small, as yet unspecified number of military advisers.

Syrian security forces secure an area following an ISIS bombing in Damascus, June 2025

 

Stay the Course

When Trump met with Shara in May, it was the first time a U.S. president had met with a Syrian president in a quarter century. Much of the impetus behind the Trump administration’s reengagement with Syria is driven by a desire to complete a total withdrawal from Iraq and Syria—an objective that can be achieved only if the new Syrian government proves capable of countering ISIS. To that aim, Washington has prioritized counterterrorism, directly assisting in operations against ISIS and providing Damascus with intelligence that has thwarted at least eight ISIS attacks. The Trump administration has also pressured the new Syrian administration to accelerate reconstruction and national unification efforts, such as the SDF integration with the government.

How things develop in Syria will depend, at least in part, on how committed Washington remains to continuing its fight against ISIS. After a decade and a half of extreme violence perpetrated by myriad actors, both state and nonstate, Syria faces a long road toward stability under the best of circumstances. Any U.S. withdrawal would create space for ISIS and other terrorist organizations to step up their attacks, further destabilizing the country, sowing divisions among its rival factions, and even opening the door for Iran-backed Shiite extremists, including Hezbollah. If the government in Damascus falls, or if it is unable to unify the country’s minority communities, Syria will plunge back into civil war.

Rather than prematurely departing a fragmented Syria, the United States and its partners in the Global Coalition should make long-term investments in the country’s security. The U.S.-led mission has significantly degraded ISIS over the preceding decade, and U.S. forces have effectively collaborated with the SDF and other local actors on counterterrorism measures. Although ISIS remains largely defeated, Syria’s postregime transition has created an opening for the group’s return. The United States should withdraw from Syria only when Syrian forces can defend the country against ISIS and other terrorist groups. That moment has not yet arrived.

A premature U.S. departure at such a fragile moment in Syria could empower ISIS, undermining the very mission that brought U.S. forces to Syria in the first place. Washington should not follow through on its planned drawdown. Instead, it must continue to share intelligence with Damascus to foil attacks by ISIS—and step up counterterrorism training with the Syrian army, the SDF, and other local partners. Beyond that, the Trump administration should prevent actions by U.S. allies—such as Israeli strikes on Syrian territory—that risk perpetuating the chaos that ISIS seeks to exploit.

Finally, instead of terminating U.S. participation in the Global Coalition, Washington should expand the grouping to include Syria itself—a move that would enable better joint training, intelligence exchange, and operations. To that end, the United States and its partners in the Global Coalition should stipulate the conditions that HTS must meet to join. To be sure, existing members will have reservations about welcoming former jihadists such as Shara. But if ISIS is to be defeated once and for all, the new Syrian government must be involved and empowered to succeed.

 

 

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