The military operations against the Islamic State and Khorasan group in Syria have already received a lot of attention among international lawyers. The conversation has focused on clarifying a highly unsettled area of law: the law that governs the use of defensive force against non-state actors. Both in this case and more generally, international lawyers have tried to distill the best or most accurate interpretation of that law. These efforts to clarify the law are, in my view, premature and not especially useful. Precisely because the law is unsettled, multiple legal positions are in play. Any one of these positions might be treated as law by some actors but not others, or in certain respects but not others. Trying to clarify the law in the face of this contestation thus elides more than it reveals; it suppresses the ambiguities and inconsistencies that shape how the law actually plays out in specific cases.

Before diving into the Syria operations, it’s useful to outline the different legal positions that were in play when the operations began. Each of these positions had some support in the practice and secondary literature, and could plausibly be invoked or applied in Syria:

  1. Defensive force against nonstate actors is never permissible.
  2. Defensive force is permissible if:
    1. the territorial state actively harbors or supports the nonstate actors, or lacks governance authority in the area from which they operate,
    2. the territorial state is unable or unwilling to suppress the nonstate actors’ violence, or
    3. the threat emanates from the territorial state.
  3. Even if defensive force is permissible:
    1. the initial attack must be especially severe to trigger the right to use such force, or
    2. the victim state has the burden of trying to cooperate the territorial state before resorting to defensive force.

The grounds for using defensive force in (2) are best understood not as mutually exclusive but as overlapping and concentric circles; as one moves from (2)(a) to (2)(c), the right to use defensive force expands. Further, any of those grounds might be further limited by (3)(a) or (3)(b).

Given the legal terrain that existed at the time, how did states engage with the law in the Syria case? I show in this article that the Syria case accentuates three preexisting trends in this area. First, most states seem to accept—and even endorse—the idea that defensive force against non-state actors is sometimes permissible. The global reaction to the operation against the Islamic State (which is justified in Iraq’s collective self-defense) has been positive. Six states are themselves using force in Syria. Many others are either using force against the Islamic State in Iraq or otherwise supporting the general campaign against the Islamic State. All of these states presumably support the Syria strikes, which are integral to the overall mission to suppress the Islamic State. Indeed, a handful of states that are not themselves using force in Syria have expressly endorsed the Syria part of the campaign. Likewise, the UN Secretary-General and Special Envoy for Syria have both made statements that appear to condone the operation against the Islamic State. And key states that might be expected to react negatively—China, India, and Syria itself—have not done so. Thus, although the positive reaction has not been uniform, it has been undeniable. States, on the whole, appear to have moved past any absolute prohibition along the lines of position (1).

Second, states have not coalesced around a legal standard for when such force is permissible. Unsurprisingly, the United States invoked the unable or unwilling standard to justify the defensive operation against the Islamic State. Other states have declined to use that language. In fact, most seem unsure of the correct legal standard or reluctant to advance any particular standard. Netherlands Deputy Prime Minister Lodewijk Asscher characterized the situation well when he said that “[f]or military operations in Syria, there is currently no international agreement on an internationally legal mandate.” Thus, although a few states have suggested that the defensive operation against the Islamic State is internationally lawful, those states have not presented a coherent theory of the case: they have not explained precisely why the operation is lawful or under what standard the operation should be assessed. Moreover, several States that have used force against the Islamic State in Iraq—with the Iraqi government’s consent—have declined to use force in Syria, in Iraq’s collective self-defense. This behavior suggests a certain discomfort with the Syria part of the campaign.

This lack of legal conviction is especially striking because the defensive operation against the Islamic State could be justified on fairly narrow legal grounds. Most significantly, the operation is taking place in an area that is ungoverned by any recognized authority. A few actors have noted as much but have declined to articulate the ungoverned space standard as the applicable law. Moreover, the operation appears to have some “buy in” from the Syrian regime. While the United States has insisted that it did not obtain Assad’s consent or otherwise coordinate with the regime, the United States did notify Assad in advance of the strikes, and Assad chose not to resist. This buy in wouldn’t satisfy a strict consent requirement, but it might help mitigate the intrusion on Syria’s sovereignty. Finally, the Islamic State’s attack on Iraq was on a very large scale—surely severe enough to pass any threshold under position (3)(a). Again, all of these grounds for justifying the operation were available to states in the Syria case; all had seeds in the preexisting practice and literature. The fact that so many states have declined to articulate a legal justification, even as they undoubtedly support the operation, suggests that they are not yet ready to push the law in a particular direction. They presumably are conflicted or uncertain about how this area of law is or should be developing.

Third, that ambivalence should not be interpreted to mean that states are strongly committed to a strict standard on defensive force. States have generally been lax about policing defensive operations against non-state actors; most states have stayed silent even in the face of operations that reflect very expansive applications of the right to use defensive force. The operation against the Khorasan group is illustrative. The United States has been somewhat opaque about the legal justification for this operation, but any justification is contingent on facts that are questionable and on legal claims, like anticipatory self-defense, that remain controversial. Yet the global reaction to the operation against the Khorasan group has been remarkably muted. The vast majority of states has neither condemned nor supported this operation. Most states have just looked the other way.

Inevitably, some international lawyers will use the Syria case to insist that one or another of the above positions is law. In fact, none of these positions by itself represents the law. With multiple positions in play, each is simultaneously legally relevant and legally deficient. A position will be legally relevant so long as it continues to resonate with enough States and other global actors—and might reasonably be invoked or applied in specific incidents. A position will be legally deficient so long as it competes with other positions for preeminence—and is not widely accepted and treated as law. For now, the position that best captures the operational practice seems not to be widely accepted as an authoritative statement of law. And whatever position is most widely accepted as authoritative seems not to reflect the operational practice. The Syria case suggests that this struggle within the law will continue for some time.

Nevertheless, a few basic guidelines can be distilled from the Syria and other cases: many states have at least implicitly supported defensive operations where, as in Syria, the territorial state actively harbors or supports the non-state actors, or lacks control over the areas from which they operate. Further, most states passively tolerate defensive operations in a much broader range of circumstances. These latter operations might not be legitimized or validated as lawful, but neither are they likely to be condemned or treated as unlawful.