by Tess Bridgeman & Brianna Rosen Source: Just Security

Since 2014, the United States has conducted periodic airstrikes and maintained hundreds of troops in Syria as part of broader counterterrorism efforts against the self-proclaimed Islamic State (IS) and al-Qaeda (AQ). The legal justification for these counterterrorism wars, which rested on shaky grounds from the start, has become more tenuous as the conflicts have shifted, with IS’ territorial defeat in both Syria and Iraq and new actors having entered the fray. Most notably, the conflict has expanded to include U.S. hostilities against Iranian-backed militia groups supporting Syrian President Bashar al-Assad’s embattled regime, without explicit congressional authorization or any clear military objective beyond deterring future attacks. Instead, the United States and these militia groups have devolved into a cycle of “tit-for-tat” strikes. The U.S. military footprint in northeastern Syria, alongside sporadic strikes against IS in the country, risks bringing the United States into direct military confrontation with pro-Assad forces, including Iran and potentially Russia. In light of these risks, it is crucial to reconsider U.S. strategic objectives in Syria, the legal basis for the use of force there, and whether U.S. military operations within Syria are the best means of meeting U.S. goals.
How U.S. Involvement in Syria’s Wars Began
The Syrian Civil War broke out in March 2011 following brutal government crackdowns against pro-democracy protests inspired by Arab Spring demonstrations against oppressive rulers throughout the Middle East. As violence erupted across the country, hundreds of rebel factions emerged. Terrorist groups, including IS and AQ, took advantage of the chaos and joined the fighting, prompting concerns in the international community about the potential for Syria to become a haven or breeding ground for terrorist threats. The United States initially responded to the civil war between the Assad regime and rebel forces in Syria by supplying rebels within the Syrian Free Army with non-lethal aid and intelligence support. It was later reported that the CIA had also established an extensive program to train and equip the rebels fighting Assad, which reportedly was terminated years later by President Donald Trump.

By early 2014, IS had emerged as a serious military threat, taking and holding territory both in Syria, where it declared Raqqa as the capital of its so-called caliphate, and neighboring Iraq. While reportedly still supporting Syrian rebel groups against Assad, the U.S. focus largely shifted toward counterterrorism goals. Congress authorized an overt program under Department of Defense (DoD) auspices which, while conceived with both counterterrorism and counter-Assad goals (such as “setting the conditions for a negotiated settlement to Syria’s civil war”), became aimed exclusively at forming, training, and equipping local groups that could conduct missions against IS and AQ in Syria. It met with limited success.

In Aug. 2014, with IS making rapid territorial gains and committing heinous atrocities, the Obama administration took the first steps toward a direct U.S. military campaign. Its first airstrikes against IS were to protect Americans in Erbil and support Iraqi forces attempting to recapture Mosul Dam. The next reported strikes were to “address the humanitarian crisis” of Yazidi civilians trapped by IS on Mt. Sinjar in Iraq.

A month later, President Barack Obama announced his intention to launch a full-scale counter-IS campaign in both Iraq and Syria. At first, U.S. operations in Syria were conducted from beyond Syria’s borders. But in 2015, the United States sent a small contingent of ground troops to train, advise, and assist local Kurdish groups in what became known as the Syrian Democratic Forces (SDF) in the fight against IS.

Obama’s stated goal for the U.S. military mission was to “degrade, and ultimately destroy, ISIL through a comprehensive and sustained counterterrorism strategy.” In addressing the American people, he explained that IS posed a threat to “American citizens, personnel and facilities” in Iraq, Syria, and “the broader Middle East,” but also admitted that no plotting had been detected against U.S. territory. However, “if left unchecked,” he continued, “these terrorists could pose a growing threat beyond that region, including to the United States.”

The battlespace in Syria was messy before the United States intervened. It grew more complex as an array of state and non-state actors took sides in Assad’s civil war and terrorist groups also began vying for territory and control. Since 2016, the United States has deliberately (but still under the heading of self-defense) and inadvertently targeted a range of other actors in Syria beyond IS and AQ, including Syrian government forces, Iranian-backed militia groups such as Kait’ib Hezbollah (KH) and Kait’ib Sayyid al-Shuhada (KSS), and Russian mercenaries.

The Trump administration escalated hostilities with Syrian government forces and their allies as it sought to more aggressively counter Iranian-backed groups in Syria as part of its “maximum pressure” campaign against Tehran. On April 6, 2017, the United States conducted a missile strike on Shayrat Airbase in response to Assad’s use of chemical weapons against his own people, the first time that the United States deliberately attacked Syrian government forces (nearly a year later, in April 2018, the United States, United Kingdom, and France conducted a second strike in response to Assad’s continued use of chemical weapons). That strike marked the start of an increasingly confrontational posture between the United States and the Syrian government and its supporters. By June 2017, there had been a series of hostilities between U.S. forces and those supporting Assad’s regime, including confrontations involving Russian aircraft and “Iran-directed forces” near the “deconfliction zone” established between the United States and Russia in 2016 around the U.S. outpost at Al-Tanf, an area in southeast Syria strategically located along the Damascus-Baghdad highway at the intersection of the country’s borders with Jordan and Iraq, which could serve as a potential landbridge for Iran to move personnel and equipment through the country to threaten Israel from the Golan Heights or elsewhere

The fighting around Al-Tanf’s “deconfliction zone” raised a series of troubling questions: “is taking and holding Syrian territory sought by Assad and his allies lawful? And, if so, what are the limits of action the United States can take against pro-regime forces under its theory of self-defense against IS?” More broadly, did the executive branch intend to wade this far into Syria’s civil war or use it as a proxy battleground without congressional authorization? In Jan. 2018, it looked as though the Trump administration had answered that question in the affirmative. Sec. of State Rex Tillerson announced the United States would continue the fight in Syria beyond the defeat of IS to ensure that neither Assad nor Iran retook newly liberated areas. At the same time, the administration increased operations against Iranian-backed militia groups, a key objective for then-National Security Advisor John Bolton.

But Trump’s escalation of hostilities was reversed abruptly on Dec. 19, 2018, when he ordered a “full” and “rapid” withdrawal of U.S. ground troops from Syria. In his defense of the decision to withdraw, Trump argued that the United States had largely defeated IS and no longer had a reason to stay in Syria, which he described as a country of “sand and death.” But the decision sparked outrage in domestic policy circles and consternation amongst coalition partners, who feared the United States was abandoning its Kurdish and Syrian allies to IS, as well as ceding strategic gains to Russia and Iran (and indeed, as U.S. forces withdrew from areas in the north, Russian forces swept in “to fill the void”). Amidst growing pressure from Congress, and following a series of high-level resignations, the administration announced it would leave behind a small contingent of U.S. troops indefinitely. Thus, even as U.S. troops were withdrawing from Syria, new forces were deploying to the north and southeast to protect oil fields, Syrian Kurdish allies, and strategic land routes from IS and pro-Syrian government forces.

A permanent contingent of U.S. troops was stationed at the garrison in Al-Tanf, which still serves as a foothold for U.S. military operations against IS and Iranian-backed militia groups. Proponents of maintaining the U.S. presence at Al-Tanf argued it was a low-cost means of preventing an IS resurgence, disrupting the Syrian economy, gaining leverage in future political negotiations, and countering Iranian influence by preventing the emergence of a landbridge running from Iran through Iraq to Syria and the Mediterranean coast, posing a potential threat to Israel. The continued U.S. presence at Al-Tanf, however, poses escalatory risks. The garrison has been attacked or nearly attacked on several occasions by Syrian, Russian, and Iranian-backed forces, despite the U.S.-Russia agreement to establish a 55-kilometer “deconfliction zone” surrounding the base. The U.S. occupation of Syrian territory without the consent of the government has also incurred reputational costs concerning U.S. respect for international law.

Current U.S. Involvement in Syria
The Biden administration inherited a complex and shifting battlefield in Syria. While Assad has regained control of large swathes of the country from rebel forces, several rounds of U.N.-brokered peace talks have proved unsuccessful in ending the civil war. The conflagration of conflicts, meanwhile, create a potential flashpoint for broader military escalation, with Iranian, Russian, Turkish, U.S., and other counter-IS coalition forces all maintaining a military presence in the country, in addition to frequent Israeli strikes against Iranian-backed forces.

At present, Biden’s approach to Syria does not appear to be markedly different from his predecessors. After concluding a lengthy policy review, senior Biden officials have indicated that the administration has four objectives in Syria: (1) reducing violence; (2) maintaining pressure on IS through a sustained military presence in eastern Syria; (3) addressing the humanitarian crisis; and (4) supporting Israel’s right to defend itself (a reference, presumably, to the Iranian-backed forces that have threatened Israel along its border with Syria from the Golan Heights). Despite expectations that the administration would wind down U.S. military operations in Syria, as it did in Afghanistan, the United States maintains a small contingent of troops and continues to conduct limited counterterrorism missions.

While the Biden administration has publicly backed away from the explicit Trump-era goal of countering Iranian influence in Syria, it nevertheless has sought to defend U.S. troops in Syria and Iraq from attacks by Iranian-backed forces. In his first overt military attack in office, President Joe Biden in Feb. 2021 ordered airstrikes against Iranian-backed militia groups in eastern Syria in response to attacks against U.S. and coalition personnel in Iraq. In a letter notifying Congress of the operation, the Biden administration claimed the militia groups posed a threat to U.S. forces and were “engaged in ongoing planning for future such attacks.” The Biden administration also submitted an Art. 51 letter to the U.N. Security Council in which it argued that, consistent with the U.N. Charter, the Feb. 2021 attack was an exercise of the United States’ “inherent right of self-defense.” The United States conducted additional strikes against the same groups in June 2021, again citing militia involvement in a series of attacks involving explosive-laden unmanned aerial vehicles (UAVs) that have targeted U.S. facilities in Iraq and Syria (and filing a second Art. 51 letter). In a speech in Nov. 2021, Sec. of Defense Lloyd Austin warned the United States would continue to counter Iran’s use of attack UAVs in Syria, which he characterized as a “constant threat to American troops, and a hindrance in the fight against [IS].”

The cycle of strikes and counter-strikes against U.S. forces in Syria and Iraq, however, suggest that U.S. military actions to date have not adequately deterred Iranian-backed forces. Al-Tanf was targeted by UAVs and indirect fire again in Oct. 2021, in what the Pentagon called a “deliberate and coordinated attack.” And in Jan. 2022, Iranian-backed groups conducted a suicide drone attack on U.S. forces at Al-Tanf and an attack on U.S. and partner forces in northeast Syria.

Apart from strikes on Iranian-backed groups, the Biden administration also has continued to conduct limited counterterrorism operations in Syria against IS. In late Jan. 2022, the U.S. military backed Kurdish-led SDF forces in an operation to regain control from IS of the Hasaka prison in northeast Syria, the largest attack that IS had committed since 2019. After more than a week of intense fighting, hundreds of IS members were killed, even as hundreds more fighters and at least 400 IS prisoners are believed to have escaped across the porous border between Syria and Iraq. Days later, on Feb. 3, 2022, Biden announced that U.S. special forces in northwest Syria had killed the leader of IS, Abu Ibrahim al-Hashimi al-Qurayshi. The killing of al-Qurayshi, as well as the territorial losses that IS has suffered, suggest the IS strategy has turned toward long-term, low-intensity insurgency, characterized increasingly by guerilla-style attacks, which should prompt serious consideration as to whether a U.S. military presence is capable of materially improving the security situation and, indeed, whether it should continue at all.

The Legal Basis for U.S. Operations in Syria
AQ and IS:
Domestic Law. On Sept. 23, 2014, Obama sent two notifications to Congress pursuant to the War Powers Resolution (WPR) requirement to notify Congress within 48 hours of introducing U.S. armed forces “into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances.” One of these “48-hour reports” concerned “a series of strikes in Syria against elements of al-Qa’ida known as the Khorasan Group,” which was situated as a component of the broader U.S. military effort “against al-Qa’ida and associated forces” in “a variety of locations.” The other report described the launch of “a systematic campaign of airstrikes and other necessary actions against [IS] terrorists in Iraq and Syria” – the beginning of the full-scale counter-IS coalition campaign that continues today.

The administration claimed the 2001 Authorization for Use of Military Force (2001 AUMF), the congressional authorization for the use of force against those responsible for the 9/11 attacks, authorized the strikes against the Khorasan Group. Since the start of the Obama administration, the executive branch has operated under the controversial legal theory that the 2001 AUMF covers not just the Taliban and AQ, but also “associated forces” of those groups. While the Sept. 2014 WPR report was vague as to whether the Khorasan Group constituted part of AQ or was considered an “associated force” of the group, either would have fit within this executive branch interpretation of the 2001 AUMF.

What was more novel, as a domestic law matter, was the claim that the counter-IS campaign in Syria and Iraq was authorized by both the 2001 and 2002 AUMFs. This was notable for three reasons, all of which point to increasingly broad and unchecked assertions of power by the executive branch over matters entrusted principally to Congress by the Constitution.

First, the Obama administration had been striking IS in Iraq since Aug. 8, 2014, but had consistently cited the president’s authority under Art. II of the Constitution, not either AUMF, as the domestic legal basis for the operations. And while it engaged in “discrete mission reporting” for several additional strikes over the next month, there was a strong argument to be made that each of those operations formed part of the same set of “hostilities” that began on Aug. 8. Based on the WPR’s requirement that any introduction of U.S. forces into hostilities on the President’s unilateral Art. II authority be terminated within 60 days unless Congress authorizes the military operations in the interim, that 60-day “termination clock” was arguably ticking ever closer to expiration by mid-September. The White House stated that the WPR’s 60-day clock did not play a role in its decision to switch to an AUMF-based domestic legal rationale for its counter-IS operations. But regardless of its intentions, by shifting to legal justifications based on pre-existing statutory force authorizations, and given little push back from a Congress that showed no interest in taking politically-fraught votes on whether to explicitly authorize the war, the administration avoided the 60-day clock cutting short its counter-IS campaign.

Second, this was the first invocation of the 2001 AUMF as a source of legal authority to use force against a group that was not described by the executive branch as “part of” or an “associated force” of AQ or the Taliban. Indeed, IS had nothing to do with the 9/11 attacks, having come into existence years after they occurred, and was itself engaged in sporadic fighting against AQ after a public break with the group. So how could the statute Congress passed to target those responsible for 9/11 authorize a new campaign against a group waging war against Iraq and Syria in 2014? According to the Obama administration, the 2001 AUMF had “authorized the use of force against the group now called [IS] since at least 2004” based on IS leader Abu Mu’sab al-Zarqawi’s close ties to Osama bin Laden in the 2004-06 timeframe (when al-Zarqawi’s group operated under the name al-Qaeda in Iraq, or AQI). Starting in those years and until 2011, the theory goes, U.S. hostilities against AQI ostensibly were covered by the 2001 AUMF (although the Bush administration had relied on a different force authorization, the 2002 AUMF, for its invasion of Iraq in 2003). The group’s public split from “current al-Qa’ida leadership” in 2014 “did not remove ISIL from coverage under the 2001 AUMF.” In essence, the Obama administration claimed that a terrorist group that joined AQ for a time while the U.S. had a troop presence in Iraq, and then went its separate ways again and even openly engaged in hostilities against each other, was still covered by the 2001 AUMF by virtue of its former relationship with AQ leadership.

This novel interpretation of the 2001 AUMF – already subject to criticism for executive branch interpretations that extended it to cover “associated forces” of AQ and the Taliban that were not described in the text of the statute and emerged long after 9/11 – has been the subject of serious debate. As Brian Finucane and Steve Pomper recently explained, the Obama administration legal team’s use of the “legally available” standard likely allowed the “decision to deem ISIS covered by the 2001 AUMF,” given “none of the senior lawyers regarded this to constitute the best interpretation of the statute.” Indeed, whether it is legally available at all is contested outside of the executive branch.

To date, however, the executive has been able to circumvent judicial review of the issue by successfully avoiding a merits hearing in Doe v. Mattis, a habeas case brought by a dual U.S.-Saudi citizen held by the U.S. military in Iraq as an IS “enemy combatant.” Congress has continued to appropriate funds for the counter-IS campaign, in what the executive described in 2016 as an “unbroken stream of appropriations” totalling $5.6 billion for counter-IS operations in Syria and Iraq. But Congress has never affirmatively authorized the use of force against IS, despite entreaties from then-President Obama to pass an IS-specific force authorization, instead essentially acquiescing to successive presidents as they have relied on increasingly broad legal interpretations of the 2001 AUMF.

Third, the reference to the 2002 AUMF as a source of authority for operations against IS was also a novel – and extremely tenuous – application of a previous statutory grant of authority. That authorization, called the “Authorization for Use of Military Force Against Iraq Resolution of 2002” (emphasis added), had one clear target – Saddam Hussein’s Iraq. Just a few months prior, in July 2014, the Obama administration had reiterated to Congress that the 2002 AUMF was “no longer used for any U.S. government activities,” and advocated its repeal (a sort of concession for failing to make progress on the president’s promised effort to “refine, and ultimately repeal” the 2001 AUMF). Yet in a 2016 report, the administration claimed that, while the primary purpose of the 2002 AUMF was to address the threat posed by Hussein, the statute, “in accordance with its express goals, has always been understood to authorize the use of force for the related dual purpose of helping to establish a stable, democratic Iraq and of addressing terrorist threats emanating from Iraq.” Accordingly, the administration claimed, the 2002 AUMF also provided ancillary authority to target AQ and IS in Iraq and, “to the extent necessary to achieve these purposes, elsewhere,” including in Syria. In the hopes of shoring up a weak argument that the 2001 AUMF covered IS, the Obama administration breathed new life into an even more tenuously-related AUMF that should have been seen as a dead letter.

International Law. Shaky domestic legal underpinnings for expanding the counter-IS campaign into Syria were matched by contested assertions of legality as a matter of international law. On the same date that the Obama administration notified Congress of its launch of the counter-IS campaign, the United States notified the U.N. Security Council – as is required under Art. 51 of the Charter, that IS and other terrorist groups operating out of Syria posed a threat, not only to Iraq, but also to the United States and its allies. The Obama administration contended that the use of force against AQ in Syria was an exercise of national self-defense, while its strikes against IS in Syria were necessary for the “collective self-defense of Iraq (and other States).” In 2016, the administration explained that “the Government of Iraq has asked the United States to lead international efforts to strike [IS] sites and strongholds in Syria in order to end the continuing armed attacks on Iraq, to protect Iraqi citizens, and ultimately to enable Iraqi forces to regain control of Iraqi borders.” Iraqi consent was clearly a sufficient basis for the otherwise lawful use of force against IS within Iraq’s borders. But the lack of consent by the government of Syria to use force within its territory posed more difficult questions – indeed, Syria stated that any strikes conducted on its territory without its consent would be considered acts of aggression.

To surmount this hurdle, the Obama administration (and subsequent administrations) argued that Syria was “unable or unwilling” to counter the threat posed by IS and AQ. The “unable or unwilling” theory holds that a victim state (the United States, on behalf of itself and Iraq) can use force in self-defense against non-state actors (IS or AQ) located in a different state (Syria) without that state’s consent, so long as the territorial state is “unable or unwilling” to effectively address the threat posed by the non-state actors. The scope and content of this theory, and indeed, whether there is sufficient opinio juris and state practice to determine its status as a rule of customary international law, remains deeply contested by other states and scholars.

The U.S. government, moreover, has never publicly articulated its views on the limits of the “unwilling or unable” doctrine. Past official statements have attempted to justify only the initial application of the theory, not its continuing application to military operations that, in some cases, have taken place nearly a decade later. As one of us has argued, there are many circumstances in Syria which indicate that it is long past time for the United States to revisit whether the “unable or unwilling” standard is still being met. For example, the capabilities of the Syrian state have changed dramatically since 2014, as it has now regained control of most of its territory and partnered with other capable nation-state military forces (Russia, for example, began its full-scale intervention in Sept. 2015), while the threat posed by IS has also changed as it has largely been defeated militarily. Are U.S. operations within Syria still necessary and proportionate to address the threat posed by IS (to Iraq and the United States)? What objectives can lawfully be pursued on that basis? U.S. Senators have received unsatisfactory answers from past administrations when they have raised similar concerns. Given the passage of time and major shifts in the conflict since 2014, the Biden administration should, at a minimum, articulate why it believes the standard is still met.

“Iranian-backed Militias”
Domestic Law. To date, there has been no congressional authorization, or even significant debate, on U.S. military operations against Iranian-backed militia groups in Syria. The Trump administration relied on a notion of ancillary self-defense under the 2001 and 2002 AUMFs to provide a legal basis for attacks on Syrian and pro-Syrian government forces that it deemed a direct threat to counter-IS operations, including Iranian-backed militias. In 2018, the administration described the strikes against such forces as “limited” and “lawful” measures to counter immediate threats to U.S. and partner forces while engaged in the campaign against IS. (Ryan Goodman and Monica Hakimi explained this interpretation here and here in 2017). The Trump administration appeared to rely on similar reasoning in its justification for the Jan. 2020 drone strike against Iranian Qods Force Commander Qassem Soleimani in Baghdad, which it claimed was authorized under the 2002 AUMF because Soleimani posed a threat to U.S. forces in Iraq.

The Biden administration’s position on how far to stretch ancillary self-defense under the 2001 and 2002 AUMFs remains unclear. Similar strikes that it has carried out in Syria thus far have been justified under Art. II of the Constitution, rather than relying on strained claims of statutory authority, and the Biden administration appropriately submitted notifications to Congress pursuant to the WPR in these cases. But a different WPR question may be lurking. Does the cycle of attacks between U.S. forces and Iranian-backed militias in Syria suggest that the United States is engaged in a pattern of hostilities that would require termination 60 days after the initial hostilities report absent congressional authorization? This is an issue to watch going forward, at least so long as the U.S. maintains a ground presence in Syria.

International Law: As a matter of international law, the Biden administration has relied on an invocation of individual self-defense and the still-contested “unable or unwilling” theory noted above as the basis for the use of force against Iranian-backed militias in Syria. But in its public statements, it has stopped short of claiming an imminent threat of attack by the Iranian-backed groups, instead citing “ongoing planning” for future attacks. This raises serious questions about whether the strikes were truly “necessary” measures of self-defense, or unlawful retaliation.

As Oona Hathaway argues, “past attacks combined with vague future planning is not sufficient to meet the necessity standard. Instead, there would need to be evidence that the groups were preparing for imminent attacks, not some possible attacks at some point in the future that they may or may not carry out.” Adil Haque agrees, calling the strikes “expressive,” not “defensive.” But as Ryan Goodman has explained, “the right of self-defense under international law—when it comes to use of force in response to a series of attacks—is, at a minimum, less clear-cut than some critics have suggested.”

Response to Assad Regime’s Chemical Weapons Use:
Domestic Law. Wholly apart from the counter-IS campaign, the Trump administration engaged in overt military attacks against the Assad regime, in 2017 and 2018, in response to its use of chemical weapons in the long-running civil war. The domestic legal justification for Trump’s two strikes further stretched already elastic claims of unilateral presidential authority to make war on foreign states absent congressional authorization. As one of us explained last year, Trump’s strikes against Syria:

arguably represent some of the farthest reaches of claimed unilateral presidential authority to use force abroad (at least since the George W. Bush administration). DOJ’s Office of Legal Counsel (OLC) opinion finalized a month after the 2018 strike claims extraordinarily broad constitutional authority to use force absent congressional authorization. The two-part test that OLC has developed over several decades to determine whether the president may use force unilaterally requires assessing whether the use of force: (1) would be in the “national interest;” and (2) “would not rise to the level of war in the constitutional sense.” Part 1, according to the opinion itself, is “a question more of policy than of law” and OLC’s purpose in including the inquiry is “not to evaluate” the actual interests at stake by some constitutional standard but to “set forth the justifications for the President’s use of military force and to situate those interests within a framework of prior precedents.” It is, as a legal matter, essentially superfluous. Part 2, intended to preserve Congress’ constitutional authority in bringing the nation to war, has also been stretched so far that short of a ground war with the likelihood of substantial U.S. casualties, Congress’ constitutional role can safely be ignored.

Such a broad interpretation of the president’s Art. II authority makes a compelling case for Congress to prioritize efforts to reform the War Powers Resolution, which remains lawmakers best hope of reigning in unauthorized wars.

International Law. It is difficult to make an argument that these strikes – conducted by the United States unilaterally in April 2017 and in coalition with France and the United Kingdom in April 2018 – were permissible uses of force under international law. The weight of opinion is rather that these strikes violated the prohibition on the use of force enshrined in article 2(4) of the U.N. Charter and customary international law. Only one state, the United Kingdom, asserted the legality of the April 2018 strikes, although others expressed various levels of political support given the heinous nature of the Assad regime’s actions (notably, when the United States released an opinion explaining its domestic legal rationale, it did not so much as mention international law). It is worth noting as well that the Obama administration’s threat of the use of force in similar circumstances during the summer of 2013 likely also violated these prohibitions, which extend to the “threat or use of force” (although Obama eventually pursued a diplomatic arrangement that succeeded in ridding Syria of the vast majority of its chemical weapons arsenal rather than following through on the unlawful threat of force that would likely have had only symbolic effects, at best, on the ground).

In a conflict characterized by brutality on many sides, including egregious violations of international humanitarian law too numerous to count, many in the U.S. foreign policy community have pointed to a moral or strategic imperative to strike back against Assad. But sadly, while the U.S. strikes against Assad have not protected Syrian civilians from the unmitigated horrors of war, they have contributed to the further erosion of bedrock rules of the international system against the use of force in international relations.

What Now?
The Biden administration has an opportunity to turn the page in Syria. Three years after U.S. officials proclaimed IS’ territorial defeat, the United States continues to conduct counterterrorism operations in Syria with no clear objective beyond securing the “enduring defeat” of IS. A compelling moral and policy case can be made that in 2014, U.S. intervention was necessary to protect civilians from IS’ brutality and help local forces take back areas it had controlled. To a large extent, these goals were met because of U.S. involvement and the contributions of coalition members organized by the United States. But the U.S. track record of securing the “enduring defeat” of an insurgent group with varying degrees of local support and a willingness to use guerilla tactics while waiting out what it sees as foreign invaders is, to put it generously, not nearly so strong. Afghanistan is only the most recent case in point. Add to this the nebulous goals and serious risks of further military confrontations with other states and hostile militia groups, and it becomes increasingly difficult to make as persuasive a case for staying in Syria as there was for the initial U.S. intervention.

The costs of maintaining these operations in Syria from a rule of law perspective also should not be minimized. While claims of collective self-defense of Iraq have been rock solid, continued reliance on the “unable or unwilling” theory of self-defense to use force within Syria – and indeed, to occupy Syrian territory that the government seeks to wrest back – is a highly tenuous proposition. Perhaps even more stark, the shaky self-defense rationale for U.S. strikes against Iranian-backed militias that appear retaliatory in nature and the all too clear violations of Art. 2(4) of the U.N. Charter that occurred during the Trump administration’s direct strikes against the Assad regime have made Syria a focal point for the erosion of the jus ad bellum. From a domestic law perspective, increasingly expansive executive branch interpretations of the 2001 AUMF to cover IS and other groups has now become the norm, with U.S. operations against groups not responsible for the 9/11 attacks (IS and its branches) eclipsing groups that were (core AQ and the Taliban).

Finally, a set of countervailing concerns could counsel in favor of maintaining some form of limited military support to U.S. non-state partners in Syria, notably the Kurdish forces who bore the brunt of the burden of liberating Syrian territory from IS. Any continuing military support to these groups, however, should be defensive in nature, its counter-IS goals should be clear, and it should solely be aimed at protecting against IS resurgence in areas that would be more vulnerable to attack should the United States decide to wind down its involvement in the conflict. And the United States must articulate how any military support even in such a limited scenario meets the standard for lawful self-defense.

The Biden administration’s policy objectives in Syria are laudable. But it remains unclear whether any of them can be met through maintaining a U.S. military presence in the country, for how long doing so remains lawful (even if it was at the start), and whether viable alternative strategies exist to meet these goals. Those are the next questions the Biden administration must urgently address.