Above: The guided-missile destroyer USS Ross fires a Tomahawk land attack missile on April 7, 2017 at a Syrian military airfield in retaliation for a chemical attack against Syrian civilians, the first direct U.S. assault on Syria and the government of President Bashar al-Assad in the six-year war there.

With the Islamic State of Iraq and the Levant (ISIL) on the retreat in Syria, hostilities have been escalating between pro-Syrian government forces seeking to regain control of Syrian territory and the United States and its partners who are in Syria with the stated purpose of fighting ISIL.  The merging of the anti-Assad and anti-ISIL conflicts has come ever closer, perhaps most dramatically on February 7 when dozens of Russia-linked pro-Syrian regime forces were killed by U.S. forces when they attacked a position held by the U.S. and its non-state partners in eastern Syria.  As an already complex battlespace becomes even murkier, so too do the goals of the United States in Syria.  The Department of State announced earlier this year that the “full and complete defeat of ISIS” is a necessary condition for ending U.S. military operations in Syria, signaling that U.S. troops would likely remain long after all territory ISIL once controlled has been liberated.  Even more open-endedly, the Trump administration has indicated that the U.S. military presence in Syria is also intended to hedge against Iranian influence and hasten the end of Assad’s rule.

Senators on both sides of the aisle have expressed concern that these goals exceed the bounds of what Congress authorized. As Senate Foreign Relations Committee Chairman Bob Corker said at a January hearing, “if [the future role of the U.S. military in Syria is] what you indicated, certainly the authorizations are not there for that kind of activity.”  A set of written exchanges between the Trump administration and Senator Kaine also honed in on lurking international law questions.  How long can the U.S. maintain a military presence in Syria based on the justification that Syria and its allies are “unwilling or unable” to effectively address the ISIL threat?  And what objectives can lawfully be pursued on that basis?

Responses to Senator Kaine from the Departments of Defense and State raise as many questions as they answer.  DoD stated it is “particularly focused on targeting” ISIL and al-Qaida in Iraq and Syria – begging the question what else it is “focused on” there.  (As a former Executive Branch lawyer who helped draft these types of letters in the past, I can attest that words are chosen carefully in responding to Congress, and the use of “particularly” signals there may be more going on than targeting ISIL).  The State Department’s response addressed the international legal basis for U.S. actions in Syria, but once again made the rather conclusory statement – consistent with past pronouncements of the Obama and Trump administrations – that the United States is acting in national and collective self-defense, that “necessary and proportionate” actions were initiated in 2014, and that those actions continue today.  This sheds no further light on what the government believes the limits of its authority to be. It merely implies that whatever those limits are, they have not yet been met.

As I explain in greater depth in a working paper, in order to answer the question of the limits of the U.S. assertion of self-defense in Syria, it’s important to address a dimension of the “unable or unwilling” doctrine the U.S. is relying on that has largely been overlooked to date. What needs to be included in any analysis: the continuing duty to meet the “unable or unwilling” standard for the entire duration of the use of force that it is used to justify.  In short, the United States has explained its reliance on the doctrine only in the context of stating the legal basis for its initial resort to force.  But merely asserting that the “unable or unwilling” test was met at the time of its initial invocation is not sufficient. It does not address a state’s ongoing obligation to operate in the non-consenting state only if the “unable or unwilling” standard continues to be met.  The administration should publicly articulate whether it has undertaken the inquiry to ensure it is still operating within the bounds of the “unable or unwilling” doctrine, why it believes the limits of the doctrine have not been reached in Syria, and finally, whether it believes counter-Iran or counter-Assad military objectives are lawful on the basis of Syria’s inability or unwillingness to address ISIL.

[This legal framework applies to two U.S. missions in Syria—operations against ISIL and operations against al-Qaida—but I focus primarily on the campaign against ISIL in light of the recent official statements about the scope of that mission.]

The Public U.S. Articulation of the “Unable or Unwilling” Standard

A state may use force in self-defense when subject to an armed attack or an imminent threat of armed attack, so long as it meets the international law requirements of necessity, proportionality, and immediacy.  The question of where force may be used must also be addressed, when the state acting in self-defense (the “victim state”) wants to conduct military operations against non-state actors (like ISIL) in another state (the “territorial state”). Absent UN Security Council authorization, if the territorial state is not itself willing or able to effectively address the threat posed to the victim state and does not provide consent to the victim state to operate within its borders, a clash of interests arises. The victim state’s inherent right of self-defense (Article 51 of the UN Charter) is pitted against the bedrock prohibition on the use of force against the territorial integrity or political independence of another state (Article 2(4) of the UN Charter).

In justifying the use of force against ISIL and al-Qa-ida in Syria, the Obama and Trump administrations have both publicly relied on a controversial theory. The 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 (ISIL, al-Qa-ida) in a territorial state (Syria) without that state’s consent, so long as the victim state determines that the territorial state is “unable or unwilling” to effectively address the threat posed by the non-state actors.  The scope and content of this self-defense theory, and even its existence as a rule of customary international law, remain deeply contested. The New York Times’ Charlie Savage explained the legal controversy at the time the United States asserted the theory in opening operations in Syria in 2014, and a March 2, 2018 article discusses its controversial acceptance by U.S. ally Australia.

The Obama administration consistently articulated the “unable or unwilling” doctrine as an element of the necessity prong of the jus ad bellum – the branch of international law governing the resort to force. Thus, the administration invoked the doctrine prior to using force in self-defense in a non-consenting state.  (Professor Ashley Deeks explained this approach in detail in a law review article, and Professor Marty Lederman has put forward arguments consistent with this approach in Just Security essays.) The Trump administration has not signaled a departure from this analysis.

The Continuing Duty to Meet the “Unable or Unwilling” Standard

Recent U.S. statements regarding its counter-Assad and counter-Iran goals in Syria illustrate the capacity of the “unable and unwilling” theory to be used or abused to justify ever-greater infringements of sovereignty.  This may stem in part from continued indeterminacy of the threshold standard for invoking the doctrine.  But it may also be caused, or at least exacerbated, by a failure to continue to evaluate whether the victim state is complying with the doctrine’s limitations during the course of military operations within the territorial state.

While always a danger with repeated uses of force, this can be especially problematic when the victim state believes a non-international armed conflict (NIAC) has developed – or remains ongoing – vis-à-vis the non-state armed group in question. This is because jus ad bellum requirements, once fulfilled prior to resorting to force, are often understood essentially to fall away when a state is involved in an armed conflict, leaving only the jus in bello – the law governing the conduct of hostilities – to regulate the conduct of parties to the conflict.  Professor Yoram Dinstein, for example, argues (at least in the context of inter-state armed conflict) that once a war of self-defense is lawfully commenced, jus ad bellum necessity and proportionality are no longer relevant.  To be sure, jus in bello constraints are meaningful with respect to the conduct of hostilities and the treatment of civilians, among other issues.  But the jus in bello does not address the question of legal authority to continue to use force in the territorial state.

There is at least one context, however, when it is already well understood that a state must continue to examine what are usually viewed as threshold jus ad bellum questions after resorting to force against non-state actors in another state: when a state operates within another state with limited consent (and in practice, consent is rarely, if ever, completely unlimited).  Prior to the resort to force on the basis of consent, the victim state must assure itself that the territorial state’s consent is voluntary, informed, and given in advance by an official or organ of the state with capacity to do so.  Consent may be limited, in terms of the objectives, means, or geographic locations in which force is used (among other potential limitations).  Following the initial resort to force, the basic rule is that the use of force by the victim state must not exceed the scope of the consent given.  In such circumstances, consent precludes wrongfulness on the part of the victim state, so long as the territorial state’s consent remains valid and its limits observed.  If consent is withdrawn, the non-territorial state’s forces must leave.  Failure to leave is not just a jus ad bellum violation (an unlawful use of force), it is also considered an international crime. The International Criminal Court’s recently-adopted definition of aggression, reflecting a 1974 UN General Assembly Resolution, enumerates as an “act of aggression” the “use of armed forces of one State which are within the territory of another State with the agreement of the receiving State, in contravention of the conditions provided for in the agreement or any extension of their presence in such territory beyond the termination of the agreement” (with certain caveats related to when the ICC’s jurisdiction may be exercised).

This same type of structural jus ad bellum constraint on the extent of force that is lawful in self-defense also applies when inability or unwillingness of the territorial state to address the threat is the legal foundation for operations within that state’s territory.  Just as in a consent-based scenario, where the lawfulness of the exercise of self-defense depends on certain requirements being met from the start and during the use of force within that state, a victim state in an “unable or unwilling” scenario also has a continuing duty to ensure the requirements of the “unable or unwilling” test are met subsequent to the initial resort to force within the non-consenting state’s territory.  This jus ad bellum requirement does not drop away – it has continuing relevance so long as operations in the territorial state continue on that basis.

While a victim state has a continuing obligation to meet the “unable or unwilling” test at all stages of its operations in the territorial state, it would be helpful to develop benchmarks for when a state should review its continued reliance on the “unable or unwilling” doctrine.  Many circumstances in Syria clearly indicate the United States should review whether the “unable or unwilling” standard is still met.  For example, new actors have entered the theater, the enemy has been substantially weakened, and the capabilities of the territorial state have changed.  (I’ll address the types of changed circumstances that should prompt states to review whether the standard continues to be met, and how those factors apply in Syria, in more detail in this paper).

It is possible that the United States has already undertaken the required inquiry internally.  If it has done so, its legal footing would be enhanced by articulating why it believes, on current facts, that the “unable or unwilling” standard is currently satisfied.  As former State Department Legal Adviser Harold Hongju Koh describes in “The Legal Adviser’s Duty to Explain,” a well-functioning system of international law requires states to explain to each other why they believe their conduct is lawful, and gives citizens, legislators, and the international community an opportunity to assess the legal theory offered.

The Vexing Question of Objectives – What Does it Mean to Effectively Address the Threat?

As I’ve argued in a previous Just Security article, at a minimum, it would be unlawful – on current facts – to shift the U.S. military mission in Syria to “confronting” Iran in order to prevent it from “cementing its military presence,” or to preventing Assad from regaining control of Syrian territory, as repulsive as his criminal regime is.  But even with respect to the counter-ISIL goals of the U.S. coalition, how far does the “unable or unwilling” doctrine get the United States, particularly once the enemy is substantially or gravely weakened?

In addressing how “defeat” is defined for the purposes of deciding when a “conditions-based” withdrawal should occur, DoD’s letter to Senator Kaine offered a new standard not previously articulated by the U.S. government: “ISIS will be defeated when local security forces are capable of effectively responding to and containing the group, and when ISIS is unable to function as a global organization.”  Do “local security forces” include the Syrian government, or only current U.S. non-state partners?  What does it mean to be “unable to function as a global organization,” and, while a desirable policy aim, is this the correct standard as a legal matter?

The DoD-articulated standard of “permanent defeat” of ISIL and inability of the group “to function as a global organization” are lawful, and indeed laudable, as goals for instruments of state power that are short of force.  The “permanent defeat” of ISIL may also be a lawful objective for measures using force based on state consent or UN Security Council authorization.  However, these are not appropriate objectives against which to assess whether a threat has been “effectively” addressed in an “unable or unwilling” scenario, which must necessarily be more limited when the threat from the non-state group drops below a certain threshold.  But what is that threshold?

Given the lack of determinacy of the “unwilling or unable” doctrine in general, it is difficult to say precisely what end state the U.S. may seek to achieve to effectively address the ISIL threat, or conversely, what Syria and its allies must be able and willing to do for the standard to no longer be met.  Permanent defeat is an indeterminate concept in the counterterrorism context in general:  does it mean no members of the organization exist?  Or that there are no members with the intent to use violence against the United States (or in the ISIL context, also against Iraq)?  Farther down the spectrum, does it require that there are no cells or operational units capable of carrying out large-scale or strategic attacks anywhere in the world?  And is that what is meant by DoD’s reference to inability to “function as a global organization?

Each of these goals would seem to give the victim state far too much leeway to remain in the territorial state conducting military operations on what could be a permanent basis, rendering necessity and proportionality all but meaningless.  One way to frame the inquiry that does not tip the balance of interests unduly toward the victim state would be to place the limit of the use of defensive force at the point when the mirror image of the threshold jus ad bellum standard is achieved – that is, if the non-state armed group is no longer conducting or imminently threatening armed attacks from the territorial state, the use of force in self-defense must cease.  This is a logical approach that would be more protective of the territorial state’s sovereignty interests, although it would likely be seen by some victim states as too stringent a standard to adequately protect against future attack by a non-state armed group that hides its hostile intentions and true capabilities.

[In their article “Unsatisfying Wars: Degrees of Risk and the Jus Ex Bello,” Professors David Luban and Gabriella Blum put forward a moral analysis of the question of the level of defense states can legitimately seek to attain.  Acknowledging the reality that all states must live with a certain amount of risk of attack by non-state actors, they propose the concept of “morally legitimate bearable risk” in deciding when a defending state must end the use of force.  They posit that states “may not shift risks onto outsiders once they have attained the morally legitimate bearable risk.”  This, too, is a thoughtful approach that seeks to balance the interests of those who may be subject to attack by non-state armed groups and those who suffer the greatest consequences when wars of self-defense are waged in response.]

This is a vitally important question that merits further consideration, as it determines when a use of force in self-defense must cease in an “unable or unwilling” scenario.

The United States in Syria 2018

The United States has not publicly articulated a view on whether it continues to meet the “unable or unwilling” test in Syria as of 2018, despite the number of tripwires that have arguably made the question of the limits of this doctrine a matter of urgency.  More broadly, there has been no public explanation of whether the United States believes it must undertake this inquiry on an ongoing basis as a legal matter.  This makes it all the more important that Congress and the public engage in a process of seeking to elucidate the limits of the controversial “unable or unwilling” theory and pressing the Trump administration for its views.

In order to bolster the legitimacy of its operations, and to allow for an informed public debate on the U.S. military presence in Syria going forward, the Administration should transparently address its views of the limits of the “unable and unwilling” doctrine.  It should also explain why it believes it hasn’t met those limits yet in Syria, and why its stated objectives of the “permanent defeat” of ISIL and the group’s inability to “function as a global organization” are lawful.

(Robert S. Price/U.S. Navy via Getty Images)