R2P was designed for pragmatists, not purists: to change state behaviour, not make new law or rewrite international relations theory. Since 2005 it has gathered momentum as a normative force, institutional catalyst, and framework for both preventive and reactive action. There are many grounds for optimism about its consolidation and further development in all these respects over the next decade and beyond: it is no longer possible for policy-makers to think and act as if mass atrocity crimes committed behind sovereign state borders are nobody else’s business. But, with dissension over the implementation of its Libyan intervention mandate in 2011 paralysing the Security Council’s subsequent response to atrocities in Syria, much remains to be done to recreate Council consensus over the hardest cases, those potentially requiring coercive military force. Some variation on the concept of ‘responsibility while protecting’, first advanced by Brazil in 2011, offers the most productive way forward.