Several weeks ago, the United Kingdom submitted an Article 51 letter to the Security Council providing notice that the UK was taking measures against ISIS “in support of the collective self-defence of Iraq as part of international efforts led by the United States.” The letter further stated that the UK “fully supports these international efforts, whose purpose is to end the continuing attack on Iraq, to protect Iraqi citizens, and to enable Iraqi forces to regain control of Iraq’s borders by striking ISIL sites and military strongholds in Syria, as necessary and proportionate measures.”
What is notable about the letter is that it implicitly adopts the “unwilling or unable” test. In determining whether the use of force in Syria was “necessary” to defend Iraq, the UK must have concluded that Syria itself could not or would not suppress the threat posed by ISIS, and that in such circumstances it is permissible under international law to use force in collective self-defense in a third state’s territory against a non-state actor, even when that third state is not supporting the non-state group. (It remains possible that Syria secretly has given the United States and other coalition partners consent to use force in Syria, though there have been no indications of this in the press.)
Several other facts are worth keeping in mind. First, states such as Jordan, Bahrain, Qatar, and the UAE, which also have undertaken airstrikes in Syria, presumably are relying on the same legal theory as the United States and UK. (That said, those states have not proffered clear statements about their legal theories.) Second, Iraq vocally has supported strikes within Syria. Third, Syria itself has not objected to these intrusions into its territory. In view of these developments, the “unwilling or unable” test is starting to seem less controversial and better settled as doctrine. Whether other European states ultimately commit to airstrikes in Syria will be informative; to date, states such as France, Denmark, and Belgium only have provided support to strikes against ISIS within Iraq, not Syria.