At the 2005 UN World Summit, world leaders endorsed the international principle of Responsibility to Protect (R2P), acknowledging that they had a responsibility to protect their citizens from genocide and mass atrocities and pledging to act in cases where governments manifestly failed in their responsibility. This marked a significant turning point in attitudes towards the protection of citizens worldwide.

This important new book charts the emergence of this principle, from its origins in a doctrine of sovereignty as responsibility, through debates about the legitimacy of...