26 November 2015
The prime minister has argued that there is a clear legal basis for military action against ISIS in Syria. Are the criteria for self-defence met? And does UN Security Council resolution 2249 provide or enhance a legal justification, or does it give merely political comfort?
Harriet Moynihan

Harriet Moynihan

Associate Fellow, International Law Programme


David Cameron leaves 10 Downing Street on 26 November 2015, bound for the Houses of Parliament where he made a statement to back joining international action against ISIS. Photo by Getty Images.
David Cameron leaves 10 Downing Street on 26 November 2015, bound for the Houses of Parliament where he made a statement to back joining international action against ISIS. Photo by Getty Images.


On 26 November, Prime Minister David Cameron made a statement to parliament setting out the case for UK air strikes against Islamic State of Iraq and Syria (ISIS) in Syrian territory. The legal justification he gave was the collective self-defence of Iraq and the individual self-defence of the UK. In his statement, the prime minister also referred to UN Security Council resolution 2249 (2015), adopted unanimously by the Council on 20 November, which calls upon states to take action against ISIS.

Security Council resolution 2249

The resolution represents a forceful collective statement urging action against ISIS, all the more potent given the rarity of unity on Syria among the five permanent members until now. It also provides political comfort to states like the UK that are seeking to win backing from their parliaments and public for military action against ISIS. But in the UK, after the 2003 invasion of Iraq, they are likely to want a legal justification too.

In order to provide legal authority for the use of force against ISIS under international law, a Security Council resolution would need to constitute a decision, taken under Chapter VII of the UN Charter, that states could use all necessary measures in their action against ISIS. Although resolution 2249 determines that ISIS is a ‘global and unprecedented threat to international peace and security’ and refers to ‘all necessary measures’, the language used in the operative part of the resolution is merely hortatory (‘calls upon’) and does not refer to Chapter VII. For those who are looking for specific UN authorisation for the use of force, this is not it.

In encouraging states to take action against ISIS – including the use of force– the resolution stresses that such measures should be taken ‘in compliance with international law’. It does not take a stance on the legal authority that states claim to have.  Rather, it can be translated as saying that if a state has a legal basis, compatible with the UN Charter, to take military action, and if it has capacity to do so, then it is encouraged to act. The question therefore remains: does the UK have a legal basis for military action in Syria in the absence of specific authorization from the Security Council?


The prime minister states that UK military action in Syria is legally justified on two grounds: collective and individual self-defence. He describes the ‘main basis’ of the global coalition’s actions against ISIS in Syria as the collective self-defence of Iraq. Iraq has requested international assistance to strike ISIS military strongholds in Syria and several states have responded to this request.

Second, the prime minister argues that ISIS’s campaign against the UK and its allies has reached the level of an ‘armed attack’ against them. The reference here is to Article 51 of the UN Charter, which affirms the right of self-defence when an armed attack occurs. To justify a claim to self-defence it is necessary to meet the criteria that the threatened attack is imminent and that the act of self-defence is both necessary and proportionate. If self-defence is invoked against a non-state armed group, it must be shown that the state in which the armed group is found is ‘unwilling or unable’ to prevent it from attacking other states. In assessing the criterion of imminence, the temporal element is not conclusive; other relevant factors include the gravity of the attack, the capability of the attacker and the nature of the threat, including if it is likely to come without warning.

The Security Council is the body responsible for international peace and security.  In its resolution 2249 it has determined that ISIS ‘constitutes a global and unprecedented threat to international peace and security’, a threat that ‘affects all regions and Member States, even those far from conflict zones’, and the Council has recognized that ISIS has the capability and intention to carry out further attacks’. This characterization of the new level of threat that ISIS poses − together with evidence of the realisation of that threat in the attacks in Tunisia, Paris and elsewhere – is clearly relevant to any assessment by individual states of the criteria required for self-defence, although each state should satisfy itself on the evidence before using force.

There may be concern that the threshold for the use of force in self-defence is being lowered to respond to individual acts of terrorism that should not properly be regarded as ‘armed conflict’ meriting an armed response. While this is a legitimate concern, it needs to be balanced against the fact that the attacks in question are not the sporadic acts of individual terrorists but a concerted major offensive from an organization that already holds large swathes of territory, with a significant budget, governance structures and technological capabilities.


There is no specific Security Council authorization for the use of force. The resolution assists the argument that the criteria for self-defence are met, although each state needs itself to be satisfied on the evidence that armed force is necessary and proportionate. Finally, whether there is a legal justification for the use of force is a separate question from whether the use of force is the right course to take.

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