Is military intervention justified if Syria uses chemical weapons? Je ne crois pas…

I read in the press that, according to French President François Hollande, the use of chemical weapons by Syria would be ‘a legitimate justification for military intervention’. Am I missing something here? The only case where the use of WMDs (or any other weapon) would legitimate an intervention against Syria would be if Syria would be using them against the interveners, who could then invoke self-defence. Or if the Security Council adopts a resolution under Chapter VII of the UN Charter authorising a military intervention in reaction to the use of WMDs by Syria against its own population (perhaps this is the situation Hollande had in mind). After all, according to para. 139 of the 2005 World Summit Outcome Document, states are prepared to exercise their responsibility to protect populations from their own governments only ‘through the Security Council, in accordance with the Charter, including Chapter VII’, and not unilaterally.

The use of chemical weapons, and of WMDs in general, is a violation of jus in bello, and not, per se, of jus ad bellum, which does not differentiate between types of weapons. Unless, of course, one argues that Hollande’s statement, which follows similar statements by the UK and the US, is a manifestation of a new opinio juris  of the international community which might eventually lead to the formation of a further exception to the prohibition of the use of force under customary international law. But, in my opinion, this conclusion seems still unjustified.