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.”

Ashley S. Deeks, The UK’s Article 51 Letter on Use of Force in Syria, Lawfare (December 12, 2014).