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On 2 December 2015 the House of Commons will debate a Government motion on using military force against ISIS/Daesh in Syria. The legal basis for using force is a central issue for this debate: does UN Security Council Resolution 2249 (2015) provide a new legal basis, or strengthen the existing ones? If not, are the existing legal arguments strong enough on their own?

The UK Government has relied on self-defence rather than Security Council authorisation as the legal basis for its proposed military action in Syria.

Resolution 2249

Although Resolution 2249 (2015) on ISIS/Daesh in Syria and Iraq, passed on 20 November 2015, is about the use of force, it does not clearly authorise states to use force. It seems intended to have more political than legal impact, particularly in displaying a unanimity that had previously been notably lacking.

The resolution uses some language familiar from other Security Council resolutions on the use of force:

  • it determines that ISIS/Daesh is ‘a global and unprecedented threat to international peace and security’, and
  • calls for ‘all necessary measures’ (code for using force) in compliance with international law to ‘redouble and coordinate’ existing efforts against ISIS/Daesh, Al-Nusrah Front (ANF), Al-Qaeda and other designated terrorist groups in Syria and Iraq, and ‘to eradicate the safe haven they have established’ in Iraq and Syria.

This careful wording implicitly supports states’ existing military actions against specific terrorist groups in those countries without either explicitly accepting or rejecting the various competing justifications or clearly providing a new stand-alone legal basis or authorisation for those actions.

The result is that states are likely to continue relying on the other varying legal arguments they have been using up until now, despite the disagreement between Russia and other states. What are these arguments?


The first is consent or invitation. The UK and other states have cited Iraq’s request to the UN for military help in combating ISIS/Daesh both in its territory and in Syria to justify their military action. Russia, on the other hand, argues that it is acting at the invitation of the state of Syria, represented by the government of Syria led by Bashar Assad.

States can lawfully use force in the territory of another state whose government has agreed to or invited it – but who can issue such an invitation: is it the effective or the legitimate government?


The second is self-defence. In its November 2015 response to the Foreign Affairs Committee report on extending British military action to Syria, the UK Government says that the main legal basis for its proposed military intervention in Syria is collective self-defence of Iraq, with the individual self-defence of the UK and collective self-defence of other states as additional legal bases.

International law allows states to use force in other states as individual or collective self-defence against an actual or imminent armed attack, as long as the force to be used is necessary and proportionate to the threat faced. However, when the armed attack comes from a ‘non-state actor’ such as ISIS/Daesh, based in a state that is ‘unwilling or unable’ to prevent the attack, the international law is not entirely clear.

UNSCR 2249 could impliedly support the view that using military force in self-defence against such attacks can be lawful, and it has even been read as suggesting that in this particular situation states could cite individual self-defence without needing to show that an armed attack is happening or imminent.

Humanitarian intervention?

A third possible argument might be humanitarian intervention, although this has not yet emerged as a fully-recognised exception in its own right to the prohibition on using force. The UK has been keen to promote it, and used it as part of its 2013 argument for using force against the Assad regime in Syria, but has not cited it in the current debate.

Is the UK involved in an armed conflict in Syria?

A final related point to touch upon is that there is some debate over whether the UK is already involved in an armed conflict with ISIS/Daesh in Syria.

Related briefings

Other Commons Library Briefing Papers looking at aspects of this debate include:

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