A US Withdrawal from Syria Will Reinvigorate the ISIS Terror Threat

A US Withdrawal from Syria Will Reinvigorate the ISIS Terror Threat

By Brian Carter

February 26, 2025

The withdrawal of US forces from Syria would risk reversing hard-earned gains against the Islamic State of Iraq and al Sham (ISIS). ISIS is not destroyed.  It is kept in check by the combined pressure of US forces in Iraq and Syria and their Kurdish allies. The withdrawal of US forces will remove that pressure and allow ISIS to reconstitute, likely rapidly, to dangerous levels.

The US military presence in Syria has contained ISIS and prevented the group from conducting attacks in the West—all at minimal risk to US personnel in Syria. The fall of the Bashar al Assad regime has opened new opportunities for the United States to conclude its mission to defeat ISIS. Withdrawing from Syria, on the other hand, would alleviate pressure on ISIS and enable the group to reconstitute and establish sanctuaries in Syria. That would in turn allow ISIS to threaten Iraq and plan terror attacks in the West, imposing yet another demand on US policymakers’ attention and resources. Russia and Turkey have offered to support the counter-ISIS mission, but they lack the ability and willingness to do so effectively.[1] The United States cannot depend on the others to fill the role that only American forces can.

ISIS could resurge in 12 to 24 months without a US presence in Syria.[2] ISIS has spent the past several years building its strength there, leading US Central Command to warn in July 2024 that the group is trying to reconstitute.[3] Though ISIS remains unable to control towns and villages, it can coerce support from the population and seize unpopulated areas temporarily.[4] ISIS, if left unchecked, would gradually exert greater control over the population and try to seize populated areas. The central Syrian desert, where ISIS primarily operates, is well-suited for sheltering, training, and organizing its current forces. But a resurgent ISIS would try to control population centers to access financial and military resources, find new recruits, and restore reliable communications for operations across Syria and externally. ISIS would almost certainly leverage such momentum to attack detention centers and thus free more of its members. ISIS would, on this trajectory, eventually regain the ability to plan and conduct attacks in the West from Syria.

ISIS cannot easily achieve these effects while the United States remains in Syria. US forces in Syria provide critical intelligence and logistical support to the Syrian Democratic Forces (SDF), which in turn keep ISIS contained and relatively isolated in the desert.[5] US intelligence sharing has already prevented attacks that could have dramatically improved ISIS’s position in Syria.[6] This over-the-horizon intelligence gathering would be far less effective in the event of a US withdrawal, especially considering that the SDF would be unlikely to share its well-developed human intelligence with the United States after a US withdrawal.[7]

No other actor can fill this counter-ISIS role in Syria. All other major actors in Syria are preoccupied with the post-Assad political transition. The interim government is prioritizing consolidating its position in Damascus and thus lacks the capacity to fight ISIS in central and eastern Syria.[8] The US-backed, Kurdish-dominated SDF is facing existential threats from the interim government and Turkey.[9] Ankara, for its part, is prioritizing the destruction of certain Kurdish groups along the Syria-Turkey border either by force or negotiation.[10] Russia has reportedly offered to help Damascus fight ISIS but similarly lacks the ability and willingness to undertake the sort of robust counterterrorism campaign that the United States can and does conduct in Syria.[11]

The United States and SDF still need to repatriate ISIS fighters held in detention facilities, which will be much more difficult if the United States withdraws. The SDF controls 28 detention facilities holding 10,000 ISIS fighters and around 46,000 ISIS supporters and refugees in displaced persons camps in northeastern Syria.[12] Successive US administrations have attempted to repatriate both the ISIS fighters and occupants of the IDP camps with little success due to the reluctance of third countries to take back their nationals.[13] The SDF may be forced to abandon these facilities in the event of a US withdrawal as it attempts to prevent its own collapse in the event of a US withdrawal.[14] Turkey and Damascus would not be able to take over the facilities in an orderly way unless there was an equitable deal between Damascus and the SDF. Without a deal, the SDF would presumably fight to secure its control of Kurdish areas in Syria at least. Turkey and Damascus currently appear to be focused on creating a dominant central government at the expense of the Kurdish components of the SDF instead of securing an equitable deal.[15] Turkey and Damascus would also probably have similar—if not more—difficulty repatriating the fighters and camp residents because they both have less leverage than the United States.

The fall of Assad has dramatically improved the military and political conditions in Syria, which the United States should be exploiting rather than ignoring. US policy has for years been at a dead-end in Syria partly because Assad made it extremely difficult to defeat ISIS or accomplish other stated US objectives. Assad failed to take the ISIS threat seriously and even turned a blind eye to Salafi-jihadi activity on occasion.[16] There is now a government in Damascus, however badly flawed and potentially unstable, that has a deep-rooted interest in suppressing and ultimately defeating ISIS.[17] US cooperation with the interim government already paid dividends when Damascus stopped an attempted ISIS attack on a prominent Shia shrine using US intelligence.[18] The attack, if successful, would have risked dramatically increasing sectarianism in Syria and destabilizing the country—all to the benefit of ISIS. The US must be cautious about engaging the interim government fully, however, until it demonstrates that it has the will and ability to suppress the non-ISIS extremists that formed part of the coalition that brought it to power and keeps it there. The fall of Assad has moreover reduced the Iranian and Russian positions in Syria, which have long impeded US counter-ISIS operations.[19] Iran and Russia regularly harassed US forces in Syria, requiring those forces to focus on defending themselves rather than fighting ISIS. The United States should maintain its force presence in Syria while supporting a political transition that enables the interim government and SDF to gradually establish more prominent roles in counter-ISIS operations. Such a transition would have been nearly impossible under Assad.

The fall of Assad has also exposed the US-backed SDF to renewed threats, however. The SDF is a Kurdish-dominated but multiethnic military force.[20] Long-running tensions between the SDF and Arabs throughout Syria have triggered defections from the SDF.[21] These Arab communities would not have defected from the SDF while Assad was in power because they feared the regime. Turkey and Damascus have also threatened the SDF, which they see as an outgrowth of the Kurdistan Workers’ Party and a threat to Syrian sovereignty. Both parties demand that the SDF dissolve itself or, as a last resort, face destruction.[22] The United States retains significant leverage in Syria that it can use to resolve some of these issues in negotiations, but a withdrawal would eliminate that leverage with Damascus.

The small, low-risk US presence in Syria has paid dividends for US national security at very little cost and can continue to do so without being an indefinite mission. Whether to leave Syria now prematurely amounts to a straightforward decision: should the United States risk having to divert significant military forces back to the Middle East in the future to fight a newly resurgent ISIS attacking or seeking to attack the West? A small investment now will avert that requirement in the future.

[1] https://www.understandingwar.org/backgrounder/iran-russia-and-syrian-regime-are-coordinating-expel-us-forces-syria

[2] https://www.criticalthreats.org/analysis/salafi-jihadi-movement-weekly-update-march-29-2023

[3] https://x.com/CENTCOM/status/1813332001010630840 

[4] https://www.washingtoninstitute.org/policy-analysis/tale-two-islamic-state-insurgencies-syria;  https://www.understandingwar.org/backgrounder/salafi-jihadi-movement-weekly-update-may-24-2023; https://x.com/GregoryPWaters/status/1637866993826029585?s=20 

[5] https://media.defense.gov/2023/Feb/07/2003157209/-1/-1/1/LEAD%20INSPECTOR%20GENERAL%20FOR%20OIR.PDF 

[6] https://thehill.com/opinion/national-security/3476953-relying-on-over-the-horizon-counterterrorism-increases-risk-to-civilians/; https://oig.usaid.gov/node/5221  

[7] https://thehill.com/opinion/national-security/3476953-relying-on-over-the-horizon-counterterrorism-increases-risk-to-civilians/; https://oig.usaid.gov/node/5221 

[8] https://www.washingtoninstitute.org/policy-analysis/status-syrias-transition-after-two-months

[9] https://apnews.com/article/syria-turkey-kurds-sdf-tishrin-dam-71f958b405bed575dc20942b8393e89b; https://www.washingtonpost.com/world/2025/02/13/syria-national-dialogue-ahmad-alsharaa-hts-sdf/f0fd5c9a-ea20-11ef-969b-cfbefacb1eb3_story.html; https://www.reuters.com/world/middle-east/rebels-take-north-syria-town-us-backed-group-turkish-source-says-2024-12-09/; https://www.foxnews.com/world/turkey-seeks-seeks-purge-pro-us-kurdish-force-helped-defeat-islamic-state-syria; 

[10] https://www.understandingwar.org/backgrounder/iran-update-february-3-2025

[11] https://www.understandingwar.org/backgrounder/iran-russia-and-syrian-regime-are-coordinating-expel-us-forces-syria; https://www.understandingwar.org/backgrounder/iran-update-february-17-2025

[12] https://crsreports.congress.gov/product/pdf/IF/IF11930#:~:text=Islamic%20State%20Detainees.&text=The%20SDF%20also%20retains%20custody,and%20Roj%20displaced%20persons%20camps; https://www.nbcnews.com/news/world/isis-islamic-state-syria-prisons-al-hol-camp-trump-assad-hts-rcna188710

[13] https://www.washingtoninstitute.org/policy-analysis/five-years-after-caliphate-too-much-remains-same-northeast-syria

[14] https://understandingwar.org/backgrounder/iran-update-january-24-2025

[15]  https://apnews.com/article/mazloum-abdi-sdf-alsharaa-islamic-state-kurds-6519679f1e4e51da60767391c005976f; https://www.middleeasteye dot net/news/syria-lists-conditions-end-rift-sdf; https://www.mei.edu/publications/will-damascus-sdf-negotiations-lead-agreement-or-escalation

[16] https://www.washingtoninstitute.org/media/4698; https://www.criticalthreats.org/analysis/ending-the-us-presence-in-syria-could-cause-a-rapid-isis-reconstitution-and-threaten-core-us-national-security-interests 

[17] https://ctc.westpoint.edu/twenty-years-after-9-11-the-fight-for-supremacy-in-northwest-syria-and-the-implications-for-global-jihad/; https://ctc.westpoint.edu/jihadi-counterterrorism-hayat-tahrir-al-sham-versus-the-islamic-state/

[18] https://www.washingtonpost.com/national-security/2025/01/24/us-syria-intelligence-hts-isis/

[19] https://www.understandingwar.org/backgrounder/iran-russia-and-syrian-regime-are-coordinating-expel-us-forces-syria

[20] https://www.understandingwar.org/backgrounder/road-ar-raqqah-background-syrian-democratic-forces; https://ecfr.eu/special/mena-armed-groups/syrian-democratic-forces-syria/

[21] https://www.understandingwar.org/sites/default/files/The%20Road%20to%20ar-Raqqah%20ID.pdf; https://www.understandingwar.org/backgrounder/competing-campaigns-against-isis-northern-syria; https://www.understandingwar.org/sites/default/files/Backgrounder_SyrianKurds.pdf; https://www.understandingwar.org/sites/default/files/Syrian%20Armed%20Opposition%20Forces%20in%20Aleppo_0.pdf; https://www.mei.edu/publications/will-damascus-sdf-negotiations-lead-agreement-or-escalation; https://syriadirect.org/protests-and-sdf-defections-discontent-simmers-in-eastern-deir-e-zor/  

[22] https://www.aljazeera dot com/news/2025/1/6/turkiye-says-kurdish-led-armed-groups-in-syria-will-be-eliminated; https://www.reuters.com/world/middle-east/turkey-says-joint-fight-needed-against-kurdish-militants-islamic-state-region-2025-01-26/; https://www.aa dot com.tr/ar/%D8%A7%D9%84%D8%AF%D9%88%D9%84-%D8%A7%D9%84%D8%B9%D8%B1%D8%A8%D9%8A%D8%A9/%D8%B3%D9%88%D8%B1%D9%8A%D8%A7-%D9%85%D8%B3%D8%AA%D9%85%D8%B1%D9%88%D9%86-%D8%A8%D8%A7%D9%84%D8%AA%D9%81%D8%A7%D9%88%D8%B6-%D9%85%D8%B9-%D9%88%D8%A7%D9%8A-%D8%A8%D9%8A-%D8%AC%D9%8A-%D9%88%D9%85%D8%B3%D8%AA%D8%B9%D8%AF%D9%88%D9%86-%D9%84%D8%A3%D9%8A-%D8%B3%D9%8A%D9%86%D8%A7%D8%B1%D9%8A%D9%88/3459498; https://x.com/SOHEB2019/status/1882455614984696101; https://x.com/SOHEB2019/status/1882455618210107645; https://x.com/SOHEB2019/status/1882444549672550567