The legal basis for the war against Isis remains contentious

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.

The US is leading a coalition of states against Isis targets in Syria. In addition, the US has conducted air strikes against the Khorasan group, which it claims is an “al-Qaida element” that poses a threat to the US and its allies. Because the US-led coalition is using force on the territory of another state, the operations are lawful only if they can be justified on the basis of one of the above listed grounds.

Syria has not expressly consented to military operations against Isis on its territory nor has any state engaging in the operations offered consent as their legal basis for action. It could be argued that Syria has implied consent. Indeed, a Syrian government spokesperson has reportedly stated, “we are facing one enemy. We should cooperate.”

Moreover, Syria has made no effort to interfere with the operations in spite of the US notifying it of the attacks prior to their launch. The strikes against Isis, one of the regime’s most powerful opponents, benefit the Syrian government and therefore it is reasonable to conclude that Damascus does not oppose the operations. Nevertheless, justifying the operations on the basis of implied consent would be contentious.

In a letter dated 23 September to the UN secretary general, the US noted that “states must be able to defend themselves, in accordance with the inherent right of individual and collective self-defence, as reflected in article 51 of the UN charter, when, as is the case here, the government of the state where the threat is located is unwilling or unable to prevent the use of its territory for such attacks”. With the exception of Iraq, the assertion of a right to individual self-defence by those states conducting the operations is problematic. This is because the right of self-defence is contingent on the state having been the subject of an “armed attack”.

In international law, an armed attack implies more than isolated criminal acts against a state’s citizens, however brutal. Rather, it implies hostilities of a certain magnitude that have a cross-border element. For Iraq, these conditions are satisfied. However, this is arguably not the case with respect to the other states participating in the strikes on Syria. Nor is there any evidence to suggest that Isis poses an imminent threat to those states that would entitle them to rely on anticipatory self-defence to prevent an attack.

The fact that a state enjoys a right of self-defence does not necessarily mean that it may conduct defensive operations against a non-state armed group based in the territory of another state. There are two views on this issue. Some experts take the position that operations against an armed group such as Isis require consent by the territorial state. Others argue that the victim state may engage in limited military operations to prevent further attacks if the territorial state is “unwilling or unable” to put an end to the group’s activities. Illustrating the extent to which the matter remains unsettled in international law, we are divided on the point. Iraq has clearly taken the position that Syria’s patent inability to deal effectively with Isis affords it the right to strike Syria.

As for the strikes against the Khorasan group, the existence of a right of self-defence is fact-dependent. Absent access to the underlying intelligence, it is difficult to fashion an assessment as to the legality of the operations. If the group is planning to conduct significant attacks against the US and the strikes on Syria are the only means of preventing them, the proponents of the “unwilling or unable” test would deem the operation to be lawful. Those who reject the test would come to the opposite conclusion.

As is apparent from the Iraqi and US statements to the UN, the current operations in Syria against Isis are being justified on the basis of collective self-defence. Importantly, the right of states to engage in collective self-defence operations depends entirely on the right of Iraq to act in self-defence against Isis in Syria and a request from Iraq for assistance in striking Isis there. The right of Iraq to act defensively in Syria depends on the position taken with regard to the “unwilling or unable” question.

For those who reject the notion, the derivative actions by the US and its partners is an unlawful use of force against Syria since Iraq itself is not entitled to use force there. Since it is self-evident that Syria is incapable of controlling Isis activities on its territory, those supporting the “unwilling or unable” test, such as the US and David Cameron, regard the collective defence operations as lawful. It must be cautioned that if Iraq withdrew its request for assistance, the operations would have to cease.

The views of the authors are offered solely in a personal capacity