The United States is again involved in tit-for-tat hostilities in Syria with unspecified “Iran-backed militia groups.” Between Aug. 23 and 25, fighting between the United States and these groups in eastern Syria resulted in U.S. casualties and fatalities amongst the hostile forces. As described here and here, the United States has been engaged in on-again-off-again clashes with these groups in Syria since 2017. 

One of Congress’ core means of ensuring transparency and obtaining the necessary information to make decisions regarding how the nation’s armed forces are used abroad is requiring reporting on situations that could lead to war in section 4(a) of the War Powers Resolution (WPR). Specifically, the WPR requires the president to report to Congress on the introduction of U.S. armed forces into hostilities within 48 hours, unless previously authorized by Congress, including the “circumstances necessitating the introduction,” their “estimated scope and duration,” and the “constitutional and legislative authority” for the introduction. Despite the transparency-forcing mechanism of WPR reporting, executive branch legal theories for hostilities in Syria have shifted over time, and the facts underlying them have often been murky. 

The most recent fighting involving “Iran-backed militias” in Syria prompted President Joe Biden to send his third report to Congress on the introduction of U.S. forces into hostilities (known as a “48-hour report”), along with a rare classified annex. But the public report covered only some of the hostilities in Syria and left many questions unanswered. (The latest report, along with its two predecessors, can be found in the comprehensive database created by the Reiss Center on Law and Security’s War Powers Resolution Reporting Project.) The Biden administration also notified the U.N. Security Council of its use of force in Syria, as required under Article 51 of the U.N. Charter. This “Article 51 letter,” as they are called, which argued the strikes were lawful measures of self-defense under international law, raises further legal questions including about a seemingly novel legal theory for the use of force.  

Background

The Trump and Biden administrations have repeatedly used force against “Iran-backed militia groups” in eastern Syria, but they have advanced different legal justifications for doing so. Under the Trump administration, the executive branch justified such hostilities as ancillary to the counter-ISIS campaign.  In doing so, it built on the Obama administration’s previous determination that the 2001 Authorization for Use of Military Force (2001 AUMF), which authorized the use of “necessary and appropriate” force against perpetrators of the 9/11 attacks (al-Qaeda) and those who harbored them (the Taliban), also applied to ISIS despite the split between that group and al-Qaeda. In a 2018 report to Congress the Trump administration argued that the 2001 AUMF authorized hostilities against Iran-backed militia groups and other non-ISIS entities in Syria, because “[a]s a matter of domestic law, the 2001 AUMF provides authority to use force to defend U.S., Coalition, and partner forces engaged in the campaign to defeat ISIS to the extent such use of force is a necessary and appropriate measure in support of counter-ISIS operations.” Accordingly, the Trump administration relied on shoehorning combat against non-ISIS groups into the overarching domestic and international legal justifications for the conflict against ISIS in Syria and Iraq. 

As a matter of international law, the United States has argued since the Obama administration’s September 2014 Article 51 letter to the U.N. Security Council that the use of force against ISIS within Syria, despite the lack of Security Council authorization or Syrian consent, is lawful based on the controversial “unable or unwilling” doctrine. The U.S. argument is that “States must be able to defend themselves, in accordance with the inherent right of individual and collective self-defense, as reflected in Article 51 of the U.N. Charter, when, as is the case here, the government of the State where the threat is located is unwilling or unable to prevent the use of its territory for such attacks.” On this view, a State may use force in another State’s territory in reliance on the “unable or unwilling” theory of self defense “if measures short of force have been exhausted or are inadequate to address the threat posed by the non-State actor [ISIS] emanating from the territory of another State.” The scope and content of this theory, and even whether it has attained the status of a customary rule of international law at all, remain deeply contested

Expanding on this theory, the Trump administration argued that “[a]s a matter of international law, 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.” Under this theory, “ancillary” strikes by U.S. forces conducting the counter-ISIS campaign were justified on the same self-defense rationale as the original intervention against ISIS in Syria. Because, in its view, these actions against non-ISIS actors in Syria were covered by the September 2014 Security Council letter, the Trump administration did not submit any further Article 51 letters related to the strikes on non-ISIS groups. 

The Biden administration has taken a different tack in its public legal justifications for hostilities against these groups by treating such combat as distinct from the counter-ISIS campaign, at least for domestic law purposes. In February and June 2021, the United States conducted preplanned, deliberate airstrikes against “Iran-backed militia groups” in Syria (and for the June strikes, in Iraq as well) at the direction of Biden. The White House cited the president’s authority as Commander in Chief under Article II of the Constitution to protect U.S. citizens, not the 2001 AUMF, as the domestic legal authority for these strikes. Accordingly, in a departure from Trump administration practice, it submitted 48-hour reports to Congress as required by the WPR. (The executive branch has narrowly interpreted “hostilities” as “a situation in which units of the U.S. armed forces are actively engaged in exchanges of fire with opposing units of hostile forces.”) In addition to the War Powers Resolution’s reporting requirements described above, the introduction of U.S. armed forces into hostilities without congressional authorization also triggers the War Powers Resolution’s 60-day clock for withdrawing such forces from hostilities – unless Congress has authorized them during that period.  

The Biden administration also reported each set of airstrikes in 2021 to the U.N. Security Council (the Article 51 letters are here and here). In these letters, the Biden administration justified the strikes as lawful measures of self-defense under international law. Notably, both sets of airstrikes were reportedly in response to attacks on U.S. personnel and facilities in Iraq, and some of the U.S. strikes in response were directed at locations within Iraq, while others were in eastern Syria. While the United States relies on the “unable or unwilling” theory for basing forces in Syria without the territorial state’s consent, the government of Iraq has consented to a U.S. presence in its territory. Perhaps for this reason, neither of the Biden administration’s 2021 Article 51 letters connected the United States’ uses of force to the counter-ISIS campaign or even mention ISIS, although each letter referenced the one preceding it, seemingly intending to incorporate by reference the U.S. arguments tying the lawfulness of its operations within Syria to the counter-ISIS campaign. 

An Incomplete Picture

The White House’s descriptions of hostilities in 48-hour reports to Congress are generally terse in recent administrations. But with respect to the latest combat in Syria, the unclassified sections of the report omit so much important information that, taken alone, it is uncharacteristically anemic. Filed in the late afternoon of Aug. 25, the unclassified text identifies hostilities (not by name, instead using the term “precision strikes”) on one day, Aug. 23. These preplanned “precision strikes” involved U.S. war planes targeting facilities associated with vaguely-described “Iran-backed groups” (reportedly Shia fighters from Afghanistan) in eastern Syria that the report does not mention by name. Press reporting indicates the initial strikes were specifically conducted to avoid casualties. 

But we know from statements by U.S. Central Command (CENTCOM) that the hostilities continued on the 24th and 25th, with Iran-backed groups targeting U.S. forces with rocket fire and U.S. forces responding with both fixed and rotary wing attack aircraft and artillery fire. These subsequent hostilities resulted in U.S. casualties (which appear to be minor injuries) as well as fatalities amongst the “Iran-backed groups.” No additional 48-hour reports have been provided to Congress in connection with these subsequent hostilities. Nor has the administration offered any other public legal justification for the combat on the 24th and 25th. 

The administration has not explicitly argued that its 48-hour report on the Aug. 23 strikes covered the follow-on hostilities, although it included the now boilerplate line at the end of the report that the “​​United States stands ready to take further action, as necessary and appropriate, to address further threats or attacks.” And although the executive branch uses a narrow definition of reportable “hostilities” for WPR purposes, it is difficult to imagine the administration does not believe the subsequent activity rose to that level, since the executive branch generally defines hostilities to include exchanges of hostile fire and airstrikes. Thus, it would seem that exchanges of fire such as those that occurred on the 24th and 25th, which were not previously WPR reported and which lack any obvious statutory authorization, should also be reported to Congress.  

Another possible explanation for the omission of the subsequent hostilities in the administration’s war powers reporting then, is that the administration, perhaps falling back on a Trump administration theory, believes they were statutorily authorized. The theory could be that while Biden relied on Article II authority for the pre-planned “precision strikes” on the 23rd, the subsequent exchanges of fire were justified as exercises of “unit self-defense” – a somewhat fuzzy and elastic concept the Department of Defense often relies on when responding to an attack or facing forces with “hostile intent.” Why wouldn’t such unit self defense-justified exchanges of fire themselves require 48-hour reporting given they clearly met even the executive branch’s “hostilities” definition? Under this theory, because the underlying U.S. force presence in eastern Syria is congressionally authorized — recall, while Congress has never enacted a use of force authorization for Syria, the Biden administration has not backed away from the theory that the U.S. presence there to combat ISIS is authorized by the 2001 AUMF — hostilities in unit self-defense undertaken by those U.S. forces are also covered by the 2001 AUMF, and thus do not require 48-hour reports to Congress under the War Powers Resolution. 

Unusually for a 48-hour hostilities report, the Aug. 25 letter sent to Congress includes a classified annex. Although such annexes are routinely included with the periodic reports submitted to Congress every six months, the only other publicly-known recent example of a classified hostilities report was that filed by the Trump administration in connection with the January 2020 strike on Iranian General Qassem Soleimani. It is possible that the Biden administration answered some of these questions in the classified annex. Perhaps it included information on the specific groups targeted, or mentioned the hostilities beyond the initial pre-planned strikes in its classified annex. given at least some of this information was made public by CENTCOM even prior to the 48-hour report’s submission, there may be other still classified information in the annex.  

What Do the Constitution and the WPR Require? 

Under the executive branch’s very malleable legal framework for when the President may use force without congressional authorization, the Department of Justice’s Office of Legal Counsel (OLC) will have had no trouble determining that the latest Syria strikes satisfy its two key requirements – that the strikes served an important “national interest” and did not amount to “war” in the constitutional sense based on their anticipated “nature, scope, and duration.” But Congress is not bound by these opinions, and some members may see the situation differently. Indeed, a bipartisan coalition of members of Congress sent a letter to President Biden in 2021 arguing that he was “acting in contravention of the Constitution,” which reserves to Congress the authority to “declare war.” 

And even under OLC’s framework, the use of force against another state (here, Syria) and the significant risk of escalation (including vis-a-vis yet another state, Iran), are factors that weigh towards deeming that a use of force amounts to “war” and requires congressional authorization. While the administration has justified these strikes in part for their supposed de-escalatory and deterrent value, these strikes have in practice been part of a pattern of repeated hostile confrontations. 

As these reports themselves recount, the three U.S. attacks reported by the White House are just part of an ongoing pattern of attacks involving U.S. forces and these Iran-backed groups. Now that these three reported U.S. attacks have been shown to be not in fact isolated, but part of a pattern of hostile activity, three questions arise: 

  • Does the repeated use of force against Iran-backed militia groups in Syria require congressional authorization under Article I of the Constitution? 
  • Under the War Powers Resolution, when did the 60-day clock start and when, if ever, did it stop? When accounting not just for pre-authorized “precision strikes” but also hostilities that followed (and preceded) them, are these tit-for-tat strikes “discrete” or should they be viewed as an ongoing set of hostilities? 
  • Under what circumstances does the administration rely on the 2001 AUMF for authority to use force against Iran-backed militia groups in Syria and when does it rely on Art. II of the Constitution? Does it rely on Art. II for pre-authorized strikes in national self defense, but on the 2001 AUMF when acting in unit self-defense?

International Law Questions Raised by the United States’ Article 51 Letter

As it did in February and June 2021, the Biden’s administration’s latest Article 51 letter argues that the strikes against Iran-backed militias were lawful measures in self-defense in response to “armed attacks against the United States.” The letter recounts “a series of attacks” on U.S. forces in Syria and Iraq throughout 2022 and before, including on Aug. 15, perpetrated by unspecified “Iran-backed militia groups.” It then argues that the Aug. 23 U.S. strikes were “taken to protect and defend the safety of its personnel, to degrade and disrupt the ongoing series of attacks against the United States and its partners and to deter the Islamic Republic of Iran and Iran-backed militia groups from conducting or supporting further attacks on United States personnel or facilities.” It closes by invoking the unable or unwilling doctrine, contending that “States must be able to defend themselves…[if] the Government of the State where the threat is located is unwilling or unable to prevent the use of its territory by non-State militia groups responsible for such attacks.”  

Notably, the letter contains no mention of ISIS, the ostensible reason for the U.S. presence in Syria in the first place, although it does state that the “letter supplements prior letters provided to the Council, including on 27 February 2021 and 29 June 2021, which further explain the basis for such actions in self-defence,” and in turn invoke the September 2014 letter focused exclusively on ISIS.

The letter raises several questions the United States should publicly address:

First, the letter says the U.S. attacks were “necessary and proportionate actions…directed at a facility near Dayr az Zawr, Syria, used by groups involved in these ongoing attacks for logistics and ammunition storage. This military response was taken after non-military options proved inadequate to address the threat, with the aim of de-escalating the situation and preventing further attacks.” Although it asserts the targeted facilities were “used by groups in these ongoing attacks” on U.S. forces, the Biden administration does not specifically identify the targeted groups or those responsible for the attacks on U.S. troops.   

  • Which groups are captured by this reference? 
  • If the group targeted in Syria on Aug. 23 is different from the group(s) that attacked U.S. forces, was the use of force by the United States “necessary” under the international law of self-defense?  

Second, disrupting an “ongoing series of attacks” – assuming they rose to the level of an armed attack individually or, perhaps more controversially, in the aggregate – could provide a basis for a valid invocation of self-defense. But without any information, at least in the public domain,  connecting the prior attacks on U.S. forces to the specific groups and facilities the United States attacked in response, it is impossible to determine whether the self-defense claim has merit.

  • What actions by the “Iran-backed militia groups” constituted an “armed attack” or imminent threat of one, and was the U.S. action actually necessary – and intended — to address that threat?

Third, even assuming that in principle deterring a non-state militia and its state backers is a lawful basis for the use of force in self-defense, there is the question of how such deterrence plays out in Syria.  Such a deterrence theory may be strained if the group in Syria targeted by the United States on August 23, was a different group than that which launched the drone attack on U.S. forces at al Tanf garrison on August 15, a casus belli listed in the latest Article 51 letter. 

The administration may view disrupting ongoing attacks as the legal basis, while deterrence is an additional policy reason for the use of force. But these strikes, like those before them, do not appear to have deterred. Instead, the opposite appears to be the case. As noted above, the U.S. strikes on Aug. 23 were followed by two days of further fighting between U.S. forces and these Iran-backed groups–fighting that caused U.S. casualties and killed hostile fighters. Further, in the following week, Iran attempted to seize U.S. Navy drones in both the Persian Gulf and the Red Sea.

  • What is the administration’s theory of deterrence? How does this theory of deterrence differ from unlawful retribution?

Fourth, though much of the argument presented in the latest Article 51 letter tracks the Biden administration’s earlier notifications to the Security Council, the new letter also contains what may be a subtle legal innovation: the legal theory for the Aug. 23 strikes implicitly relies on stacking multiple claims of “unable or unwilling” on top of one another. As a threshold matter, the United States justifies the use of force in Syria and its ongoing military presence in the country on the grounds that Syria is unable or unwilling to effectively counter the threat posed by ISIS. The Biden administration has now added an additional layer, by arguing that while U.S. forces are in Syria (ostensibly still focused on ensuring the “enduring defeat” of ISIS) they may also use force against “Iran-backed militia groups” because the Government of Syria “is unwilling or unable to prevent the use of its territory” by those non-ISIS groups.  

This theory, which we’ll call “unable or unwilling squared,” differs from that of the Trump administration and appears novel in at least two respects. First, we are unaware of any state previously justifying the use of force through compounding unable or unwilling claims. Second, States have previously justified the use of force against non-state actors under the “unable or unwilling” doctrine or similar formulations in connection with territorial States whose territory was being used by the non-state actor to launch cross-border attacks against another State. At least as articulated in this letter, the United States is arguing that if Syria is unable or unwilling to prevent attacks within its own borders against foreign forces whose presence in its territory is non-consensual, then U.S. forces may resort to force on that basis.  

  • How does the invocation of the “unable or unwilling” theory against Iran-backed militia groups in Syria relate to the 2014 “unable or unwilling” claim against ISIS? 
  • What is the end-point of the “unable or unwilling” theory? (Or as one of us put it back in 2018, “when does the legal basis for U.S. forces in Syria expire?”) At what point will it no longer be “necessary” to use force in Syria as a matter of self-defense?

Fifth, the Biden administration should clarify the interplay of international and domestic legal regimes applicable to the recurring combat with Iran-backed militia groups in Syria. Under the U.S. approach, although the preceding series of attacks on U.S. forces were sufficiently serious to constitute “armed attacks” under Article 51 of the U.N. Charter and thus provide a predicate for U.S. airstrikes in self-defense as a response, these attacks on U.S. forces were not reported to Congress under the War Powers Resolution. 

  • Why did earlier attacks on U.S. forces constitute “armed attacks” for the purposes of the U.N. Charter, but not the introduction of U.S. forces into hostilities for the purpose of the War Powers Resolution?

Conclusion

Moving forward, the Biden administration and Congress should work together to curtail legal theories for war-making that bypass the constraints of the U.N. Charter and the Constitution. Not only do these theories risk aggrandizing executive branch power, they also risk undercutting frameworks intended to afford fuller consideration of the costs and benefits of conflict and place a check on imprudent war-making. In short, the creative legal interpretations that enabled the expansion of U.S. hostilities in Syria should not become the norm. The ongoing hostilities in Syria illustrate how creative legal theories crafted by the executive branch and effectively unchallenged by Congress erode these constraints and may increase the risk of further conflict. 

As we and colleagues have previously written (here, here, and here), restoring institutional checks on the use of force will ultimately require structural legislative reform. In the meantime, there are more modest steps the Biden administration should take to keep the legal theories it has developed in Syria from expanding further. It should start by explaining the metes and bounds of those theories publicly, including by answering the questions we pose above.

IMAGE: US soldiers walk near Bradley Fighting Vehicles (BFVs) during a joint military exercise between forces of the US-led “Combined Joint Task Force-Operation Inherent Resolve” coalition against the Islamic State (IS) group and members of the Syrian Democratic Forces (SDF) in the countryside of the town of al-Malikiya (Derik in Kurdish) in Syria’s northeastern Hasakah province on September 7, 2022. (Photo by Delil SOULEIMAN / AFP) (Photo by DELIL SOULEIMAN/AFP via Getty Images)