Collective defence agreements, of the sort that exist between, for example, NATO members, EU members, and African Union members, are prominent deterrence mechanisms. These agreements commit their members to treating an attack on any one of them as an attack on all. Such agreements clearly have significant deterrent benefits for their members. They offer a degree of assistance that will make it very difficult for an adversary to win an aggressive war against any member. On the face of it, then, such agreements seem obviously morally permissible and, indeed, morally desirable. In this talk, I suggest that the moral picture is in fact more mixed, and that forming and acting on these agreements stands in need of justification.

Professor Helen Frowe