The hundreds of U.S. troops who are still stationed in Syria, on a mission to “maintain the enduring defeat” of the Islamic State (also known as ISIS), are fighting an off-and-on battle with Iranian-backed militias. Just last week, the United States conducted strikes against fighters described as “Iran-supported malign actors” who attacked a base housing U.S. troops in eastern Syria. It is not the first time this has happened. Since 2016, U.S. forces in Syria have fought not only members of ISIS but also Syrian government troops, Iranian-backed militias, and Russian mercenaries. This longer list of adversaries shows what a complex and perilous environment U.S. forces are operating in and raises questions about the underlying legal rationales for their deployment.

Both domestic and international laws restrict war making, and thus, in principle, the president is restrained from unilaterally using military force whenever he or she desires. U.S. law makes exceptions for cases of self-defense and conflicts that are approved by Congress, and international law similarly allows for self-defense and force approved by the UN Security Council. But with both Congress and the UN Security Council reluctant to approve conflicts for both policy and political reasons, the U.S. government has figured out creative ways to circumvent these bodies. Nowhere is that clearer than in Syria, where successive U.S. administrations have engaged in clever legal maneuvering to fit a wide range of U.S. operations within the scope of domestic laws. The original rationale for going to war in Syria in 2014 was to fight ISIS and al Qaeda. Although Congress did not authorize the campaign against ISIS in advance, it subsequently appropriated funding for U.S. counter-ISIS operations. Yet since 2016, U.S. forces in Syria have also battled a handful of other foes with no congressional authorization or even debate.

In a sense, the United States may have backed itself into a corner in Syria. Close observers of the situation (including my own organization, the International Crisis Group) argue that the several hundred U.S. troops on the ground there for counter-ISIS purposes have come to play an essential role in girding northeast Syria against what could well be a bloody free-for-all should they withdraw. Ideally, the executive branch would work with Congress on new legislation that squarely defines the contours of U.S. military activities in Syria. At the same time, there seems virtually no likelihood that the Biden administration, which believes it already has the authority it needs for these activities and has expressed its intention to maintain these deployments, will seek that authorization from Congress. Nor is there much chance that Congress, which is divided on the continued U.S. presence in Syria and has grown accustomed as a body to dodging its responsibility for matters of war and peace, will take a consequential vote on U.S. operations in Syria.

But even if the result is that the administration maintains its current course in Syria, the legal contortions the U.S. government has taken there should not be allowed to become a precedent. In order to keep this from happening, it is important to acknowledge the myriad ways in which the U.S. government has stretched its current authority and consider how a reinvigorated framework for congressional engagement might avoid seeing future administrations take similar liberties in similar scenarios.


Under international law, the UN Charter prohibits the use of force and assigns to the Security Council the responsibility for authorizing it in the service of maintaining international peace and security. The exercise of the “inherent right of . . . self-defence,” enshrined in Article 51 of the charter, is an exception to this prohibition. Any such defensive measures must be both necessary and proportionate to the threat, and they must be reported to the Security Council in what are called “Article 51 letters.”

Article I of the U.S. Constitution, meanwhile, gives Congress the power to “declare War.” Congress has exercised this power through formal declarations of war (as in 1941 against Japan) and in recent decades through the functional equivalent of statutory authorizations for the use of military force. For example, the United States has waged the war on terrorism primarily under the broadly worded war authorization passed by Congress a week after 9/11, the 2001 Authorization for Use of Military Force, or AUMF. Originally intended to allow the U.S. military to fight those responsible for the terrorist attacks (understood to include at least al Qaeda), the authorization does not explicitly specify the enemy and thus has been used by the executive branch in the 20 years since to justify U.S. military operations against an ever-expanding raft of groups, many of which didn’t even exist in 2001.

In addition to conflicts authorized by Congress, the president has some independent authority to use force under Article II of the Constitution, which names the president the commander in chief of the “Army and Navy of the United States.” Although the scope of this authority is contested, it includes defending the United States and U.S. forces from sudden attack.

In order to place limits on the president from using this authority to take the country to war on his or her own, in 1973, during the waning days of the Vietnam War, Congress passed the War Powers Resolution. Under this law, in the absence of congressional authorization, the president must report within 48 hours when U.S. forces are introduced into “hostilities” (which the executive branch interprets to include airstrikes and exchanges of fire with hostile forces) or introduced into situations of imminent hostilities. Once such a report is submitted to Congress, a 60-day clock starts ticking. By the end of the countdown, U.S. forces must withdraw from hostilities unless Congress has specifically authorized their actions.


When the Syrian civil war broke out, in 2011, the United States condemned atrocities by Syrian President Bashar al-Assad but refrained from direct military intervention in the conflict. But once ISIS launched a campaign of slaughter, rape, and enslavement against the Yazidis in Iraq and threatened U.S. personnel and the Iraqi government in September 2014, the Obama administration ordered airstrikes against the group, first in Iraq and then in Syria. A year later, President Barack Obama sent ground troops into the country to assist (in an ostensibly noncombat role) local fighters to combat ISIS. Iran and Russia had already taken the Syrian government’s side in the civil war, providing military support in the form of fighters, equipment, and airstrikes.

When the United States initiated military operations in Syria, both its domestic and its international legal justifications were tied to the idea that these activities served a counterterrorism mission. As a matter of domestic law, the Obama administration adopted the theory that Congress had already authorized the use of force against not only al Qaeda but also ISIS in Syria. Under this interpretation, the 2001 AUMF authorized U.S. military operations against ISIS on account of the ties between Abu Musab al-Zarqawi (the founder of ISIS’s predecessor organization, al Qaeda in Iraq) and Osama bin Laden—even though ISIS had publicly split from al Qaeda. Under this theory, the Obama administration didn’t need Congress to declare war in 2014 against ISIS because it had effectively already done so in 2001, despite the fact that the group had not existed then.

To cover its bases in international law, the United States sent an Article 51 letter to the UN Security Council, saying that it was acting against ISIS in Syria in self-defense—on behalf of both Iraq and itself. The United States argued that it was necessary to use force in Syria without the consent of the Syrian government, because Syria was unable or unwilling to counter the threat posed by ISIS and al Qaeda. Variations of this theory had been invoked for centuries by countries using force against nonstate actors, and other members of the counter-ISIS coalition, such as the United Kingdom and Germany, also relied upon it for operations in Syria. Although other countries such as Russia and Iran have embraced the “unable or unwilling” theory in different contexts, it remains contested by some countries, such as Mexico.

The range of U.S. operations in Syria does not fall neatly into the counter-ISIS mission.

Upon arriving in Syria, U.S. forces quickly began to exchange fire with groups other than ISIS and al Qaeda. In September 2016, the United States and coalition partners launched a series of airstrikes on Syrian government forces in Deir ez-Zor Province, mistaking them for ISIS fighters. Hostilities between U.S. and Syrian forces escalated during the administration of President Donald Trump, with much of the fighting concentrated around the military base the United States had set up in al-Tanf, in southeastern Syria. Although the U.S. troop presence there was established as part of the counter-ISIS mission, U.S. officials such as John Bolton, Trump’s national security adviser, saw it as principally aimed at countering Iran. In May and June 2017, the U.S. military repeatedly battled fighters supporting the Syrian government, carrying out airstrikes against ground forces and shooting down two drones.

Farther north, in June 2017, U.S. Navy pilots shot down a Syrian jet that had attacked the Syrian Democratic Forces, the largely Kurdish militia supported by the United States. The following February, the U.S. military fought a four-hour battle with pro-Assad forces after they had attacked U.S. troops and the SDF at a small outpost in eastern Syria. U.S. forces killed hundreds of enemy troops in the battle, many of them Russian mercenaries.

In December 2019, U.S. warplanes struck Kataib Hezbollah, a Shiite paramilitary group, in both Iraq and Syria, in response to rocket fire by Iranian-backed groups against U.S. troops in Iraq. That round of hostilities would later involve the United States killing General Qasem Soleimani, the commander of Iran’s Islamic Revolutionary Guard Corps Quds Force, and Iran attacking U.S. troops in Iraq with ballistic missiles. In August 2020, U.S. soldiers had a gunfight with Syrian troops at a checkpoint, killing one of the Syrian fighters.

Distinct from these hostilities on the periphery of the counter-ISIS mission in eastern Syria, the Trump administration also launched airstrikes against the Syrian government in 2017 and 2018 in response to its use of chemical weapons.

Fights between U.S. troops and Iranian-backed militias have continued during the administration of President Joe Biden. In February and June 2021, in response to attacks on its forces in Iraq, the U.S. military launched airstrikes against Iranian-backed militias in Syria along the Iraqi border. And in October of that year, U.S. forces at al-Tanf were hit by five so-called suicide drones, which according to The New York Times were dispatched in retaliation for earlier Israeli strikes on Iranian forces in Syria. Two months later, a British fighter jet shot down a drone approaching the al-Tanf outpost. Then, in the first week of 2022, U.S. forces in northeast Syria, acting to defend themselves and their SDF partners, attacked Iranian-backed launched strikes against pro-Iran fighters who returned fire. These incidents speak for themselves regarding the potential for escalation.


The range of U.S. operations in Syria does not fall neatly into the counter-ISIS mission the United States claimed it had authority for when it first sent forces there. In order to accommodate the inconvenient facts of U.S. combat in Syria, the executive branch has resorted to creative lawyering. Although the Obama administration did not offer a legal theory for its erroneous strikes on Syrian troops, the Trump and Biden administrations have articulated justifications for fighting non-ISIS forces, and these rationales differ in some respects.

The Trump administration shoehorned attacks against pro-Syrian government forces into the 2001 AUMF through a theory of “ancillary self-defense.” Under this interpretation, U.S. forces and their partners were undertaking a mission authorized by the 2001 AUMF (against al Qaeda and ISIS), and the AUMF therefore gave U.S. forces the right to defend themselves, even from groups not covered by the AUMF. By relying on the 2001 AUMF as authority for these strikes, rather than the president’s commander-in-chief authority from Article II of the Constitution, the Trump administration skirted the War Powers Resolution. It avoided having to report its actions to Congress and triggering the resolution’s 60-day clock for withdrawal.

Under international law, the Trump administration relied on a similar theory of ancillary self-defense. Building off the underlying claim of self-defense against ISIS, the administration argued in a report to Congress that the “necessary and proportionate use of force in national and collective self-defense against ISIS in Syria includes measures to defend U.S., Coalition, and U.S.-supported partner forces while engaged in the campaign to defeat ISIS.” In other words, the administration claimed that in acting in self-defense against ISIS, it could defend itself against any other group that might threaten it along the way.

These theories are not universally accepted, but even if one accepts them in principle, the Trump administration’s application of them to the fighting in Syria is questionable. To the extent that aspects of the underlying mission are not in fact counter-ISIS, these rationales are particularly strained. Statements by Trump himself suggesting U.S. troops were in northeast Syria primarily “to secure the oil,” and the apparent intention of some Trump-era officials to exploit the U.S. presence in al-Tanf in the course of a larger strategy designed to contain Iran, further undermined their credibility.

The creative legal interpretations that enabled the expansion of U.S. hostilities in Syria should not become the norm.

Perhaps to guard against such strains, Biden administration officials have emphasized that forces in al-Tanf have become more involved in counter-ISIS activities in recent months as the threat of potential ISIS reconstitution has grown.

The Biden administration has invoked the president’s constitutional authority to justify actions against Iranian-supported forces rather than relying on an expansive reading of the 2001 AUMF. It did this in connection with U.S. airstrikes against Iranian-backed militias in Syria in February and June 2021. In notifying Congress of these strikes under the War Powers Resolution, the administration explained that the president acted to protect U.S. personnel and deter future attacks. Although it has not explained itself, the new administration apparently takes the position that neither notification started the 60-day clock under the War Powers Resolution for withdrawing U.S. forces from hostilities.

Yet the administration’s failure to send notifications to Congress within 48 hours regarding the October drone attack on al-Tanf and the fighting during the first week of 2022, even though they seem to constitute fresh hostilities, raises questions about whether the White House is relying on an unduly narrow interpretation of “hostilities” in order to avoid triggering the War Powers Resolution’s 60-day clock. Or perhaps the administration has reverted to relying upon the 2001 AUMF as authority for such hostilities, as the Trump administration did. It is possible the administration may not be guided by any particular legal theory or interpretation but is simply picking and choosing between using the 2001 war authorization and Article II of the Constitution as authority in Syria in order to maximize operational flexibility and avoid explaining itself.

Whereas Congress arguably ratified the conflict against ISIS through subsequent budgetary appropriations, giving de facto permission by funding the fight, there has been no such congressional endorsement of the widening war against other foes in Syria. Indeed, in a November letter sent to Biden, a bipartisan group of members of the House of Representatives posed questions regarding the legal basis for recent hostilities in Syria, particularly those involving non-ISIS forces. As of early January, the White House had not responded.


Moving forward, the Biden administration and Congress need to work together to curtail legal theories for war making that bypass the constraints of the UN Charter and the Constitution, as such gamesmanship avoids adequate consideration of the costs and benefits of conflict and undercuts legal frameworks that were intended to prevent imprudent war making. The creative legal interpretations that enabled the expansion of U.S. hostilities in Syria should not become the norm.

There are some steps the Biden administration can take that would at least keep these theories from becoming even more expansive. The administration should articulate more precisely the limits of the “unable or unwilling” theory—that is, at what point it will no longer be necessary to use force in Syria as a matter of self-defense under international law. It should also more clearly spell out its definition of “hostilities” under the War Powers Resolution and why recent attacks on U.S. forces in Syria did not fit it. And it should do so in a responsible way, mindful that this definition of hostilities could be used by successor administrations. The administration should also explain whether it subscribes to the Trump administration’s theory that the 2001 AUMF provides ancillary authority to fight other non-AUMF foes and, if so, under what circumstances it relies on the AUMF as opposed to Article II of the Constitution.

New legislation is also necessary. It is long past time for Congress to reform the 2001 AUMF. To prevent future administrations from stretching the law even further, a new war authorization bill should specify explicitly who the enemy is. At a minimum, revising the 2001 AUMF would help cauterize the existing, seemingly open-ended scope of the law by foreclosing any claims of ancillary authority. Congress should also take the long overdue step of reforming the War Powers Resolution to create a responsible definition of its terms such as “hostilities” and a template for the future use of force authorizations, one that tries to steer their successors away from the overly broad language used in 2001.

Expansive precedents take root with unfortunate speed in national security law. There is already evidence that legal innovations originating in Syria won’t necessarily stay in Syria. The Trump administration relied on the theory of ancillary self-defense, which was first introduced to justify hostilities around al-Tanf and elsewhere in eastern Syria in 2017 and 2018, to justify the 2020 killing of Soleimani in Iraq. In that case, the administration claimed (along with other legal theories) that the 2002 Iraq war authorization, which unfortunately remains on the books long after Saddam Hussein’s demise, provided authority for the strike on Soleimani, an Iranian general, because he posed a threat to U.S. forces in Iraq.

Domestic and international legal constraints on the use of force are designed to promote deliberation and ensure that the costs and benefits of war are adequately weighed before one is waged. What is going on in Syria illustrates how creative legal theories cooked up by the executive branch and unchallenged by Congress erode these constraints and increase the likelihood of future conflict. To restore the limits on war making, the Biden administration and its allies in Congress should start rebuilding the safeguards that too many administrations have chipped away.

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  • BRIAN FINUCANE is Senior Adviser in the U.S. Program at the International Crisis Group and a Nonresident Senior Fellow at the Reiss Center on Law and Security at NYU School of Law. During the Obama and Trump administrations, he served as an Attorney Adviser in the Office of the Legal Adviser at the U.S. State Department.
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