There has been widespread support for the idea that the so-called international community has a remedial moral responsibility to protect vulnerable populations from mass atrocities when their own governments fail to do so. But, where exactly is this responsibility located?In other words, which body or bodies can be expected to discharge a duty to safeguard those who lack the protection of – or, indeed, come under threat from – their own government?The question becomes particularly pressing when the United Nations is unwilling or unable to act and there is no one state to fill the breach.

In this chapter, I examine ‘coalitions of the willing’ as one (likely provocative) answer to this question, and explore how the informal nature of such associations should inform the judgements of moral responsibility that we make in relation to them.Perhaps most controversially, I propose that, under certain circumstances, states and other institutional agents have a duty to contribute to forming such an ad hoc association, and then to participate in a collective endeavour – and that these circumstances may include cases of military intervention in response to mass atrocity without UN Security Council authorisation.