Created from the ashes of the Second World War with the Allies determined to prevent a repeat of Adolf Hitler's horrors, the United Nations for most of its existence has focused far more on external aggression than internal mass killings. Yet Nazi Germany was guilty of both. Unlike aggression against other countries, the systematic and large-scale extermination of Jews was a new horror. As the above quote from Secretary-General Kofi Annan suggests, the organisation is at long last elevating the doctrine of preventing mass atrocities against people to the same level of collective responsibility as preventing and repelling armed aggression against states. That journey is the theme of this book. Both sets of responsibility require judgements on when, how and how much force to use. This provides the leitmotif of my narrative: the procedural norm which emphasises multilateral forums and approaches for making the decision to use force, the substantive reasons justifying the recourse to force, and the manner in which both these embedded norms have come under pressure in recent times.

The second strand in my narrative is the distinction between legality and legitimacy. According to the Secretary-General's High-Level Panel on Threats, Challenges and Change, ‘The maintenance of world peace and security depends importantly on there being a common global understanding, and acceptance, of when the application of force is both legal and legitimate.’