Here is the UK’s statement on “the legality of military action in Syria following the chemical weapons attack in Eastern Damascus on 21 August 2013.” It maintains that “[i]f action in the Security Council is blocked, the UK would still be permitted under international law to take exceptional measures in order to alleviate the scale of the overwhelming humanitarian catastrophe in Syria by deterring and disrupting the further use of chemical weapons by the Syrian regime.” Humanitarian intervention is permitted in such circumstances, according to the UK, if three conditions are met:
(i) there is convincing evidence, generally accepted by the international community as a whole, of extreme humanitarian distress on a large scale, requiring immediate and urgent relief;
(ii) it must be objectively clear that there is no practicable alternative to the use of force if lives are to be saved; and
(iii) the proposed use of force must be necessary and proportionate to the aim of relief of humanitarian need and must be strictly limited in time and scope to this aim (i.e. the minimum necessary to achieve that end and for no other purpose).
The paper then argues that all three conditions are met.
The UK “legal position” contains not a bit of legal analysis. It does not explain how humanitarian intervention as it describes it is consistent with the U.N. Charter’s clear prohibition on the use of force absent Security Council authorization or in self-defense. Presumably to be lawful in the face of the Charter, the doctrine of humanitarian intervention must be supported by customary international law. Yet the UK does not try to explain why it believes that humanitarian intervention as it describes it represents the general and consistent practice of states followed from a sense of legal obligation. It does not try to do this, I think, because there would be no basis for such a position. So in the end this is just a UK ipse dixit that (as the paper puts it in the end) intervention is justified as an “exceptional measure on grounds of overwhelming humanitarian necessity.” I.e. the UK thinks the ends justify the means, including non-compliance with the U.N. Charter.
The UK's legal justification is poor, but that does not mean it is useless. As Eric Posner and I argued in chapter 6 of The Limits of International Law, it is always better for nations to offer a poor or weak legal justification than no justification at all. For nations that show indifference to international law when violating it are viewed in an even worse light than nations that offer weak or poor justifications for violating international law.