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On September 22, 2014, a U.S.-led coalition began airstrikes against the so-called Islamic State in Syria. At the same time, the United States started targeting the Khorasan group in Syria. These two operations raise (again) the question of when States may use defensive force against non-State actors in other States. The text of the United Nations Charter does not resolve the question. Article 2(4) prohibits States from using force “against the territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations.” Article 51 then recognizes “the inherent right to individual or collective self-defense if an armed attack occurs.” The proper interpretation of those provisions for cases involving non-State actors is uncertain and contested.

The response in the secondary literature has been to try to resolve the uncertainty — to identify the interpretation that is or should be correct. For example, two prominent expert reports on the topic, the Chatham House Principles of International Law on the Use of Force in Self-Defence and the Leiden Policy Recommendations on Counter-Terrorism and International Law, recognize that the law in this area is uncertain and controversial. Each report then aims to clarify the law’s substantive content. Similarly, Sir Daniel Bethlehem’s recent piece in the American Journal of International Law argues that much of the scholarly literature on the topic is far removed from the operational practice. Bethlehem offers a set of principles that he hopes will “attract a measure of agreement about the contours of the law.” His principles instead generated more debate. Others argued that the law is not as he articulated.

Efforts to clarify the law on the use of defensive force against non-State actors are premature. The evident ambiguities and inconsistences in the practice reflect an ongoing struggle over the law’s proper content. This struggle cannot neatly be resolved because the international legal system is, at bottom, decentralized. No actor is charged with settling competing claims on the law or dismissing invalid claims. Unless and until States gravitate toward the same claim, multiple claims will continue to circulate simultaneously. Any one of these claims might be treated as law by only some actors and not others, or in only certain respects and not others. Trying to clarify the law in the face of this contestation thus elides more than it reveals. It suppresses the underlying tensions that shape how global actors engage with the law in concrete cases.

This article takes a different approach. Rather than try to distill the best or most accurate interpretation of the law, I map the positions that were plausibly available when the Syria operations began. I do so precisely because the law in this area has been unsettled. A broad range of legal positions might reasonably be invoked or applied in any given case. After map-ping the legal terrain, I argue that the current operations in Syria accentuate three preexisting trends. First, the claim that international law absolutely prohibits the use of defensive force against non-State actors is losing legal traction. That claim is increasingly difficult to sustain. Second, States have not coalesced around a legal standard on when such force is lawful. Most States seem conflicted or uncertain on that question, and have declined to advance a particular legal position. Third, this ambivalence has contributed to a sizeable gap between the norms that are widely articulated as law and the ones that reflect the operational practice. States regularly tolerate operations that they are not yet willing to legitimize with legal language.


International Law | Law | Military, War, and Peace