Syria has not expressly consented to US military strikes against Islamic State on its territory, and justifying the operation on the basis of implied consent is controversial.
On 26 September, the House of Commons voted 524 to 43 to support military action against Islamic State (Isis) in Iraq. Although the government’s motion did not extend to operations in Syria, the prime minister asserted in his statement to the Commons that such measures would be lawful. Most, if not all, international law experts agree that military operations against Isis in Iraq comport with international law. The same cannot be said of the operations against Isis in Syria.
The use of force in the territory of another state is prohibited under the UN charter and customary international law. However, there are four universally recognised exceptions to this prohibition: UN security council authorisation (it has not acted in this case); consent of the state on whose territory the operations are conducted; self-defence; and collective self-defence. A fifth exception, and one asserted in August 2013 by the UK government with regard to the use of chemical weapons by the Syrian regime, is humanitarian intervention in circumstances involving “overwhelming humanitarian necessity”. This justification remains controversial. Even if it exists in international law, the Isis abuses have, to date, not reached the requisite threshold.