Self-defence is a crucial feature of international law, amounting as it does to the only lawful basis for the unilateral use of military force in the modern world.1 It is not surprising, then, that a vast literature has developed regarding the nature and parameters of the exercise of self-defence in international law.2 However, there has been relatively little consideration of the specific concept of collective self-defence. This is true not just in scholarship3 but also in terms of the way that states discuss or debate self-defence in a general sense.4 Marginalisation of the topic in this way perhaps has been due to the common perception that collective self-defence was effectively ‘invented’ by the drafters of the United Nations (UN) Charter,5 and the view that states have exercised it only very rarely since.6