R2P was developed under U.N. auspices in the early 2000s, and was approved by all member states at the U.N.’s 2005 World Summit. Established as a reaction the genocides of the 1990s in Rwanda and Bosnia, where international failure to act resulted in loss of millions of lives, R2P stipulates that states must take action to protect their citizens and prevent crimes against humanity within their borders. If they fail to do so, the international community has a responsibility to intervene.

The duty to intervene places an obligation on all member nations and should normally be reinforced by a U.N. Security Council resolution authorizing use of force. In the case of Syria, however, any attempt by the Security Council to act has been consistently stymied by Russia and China. But R2P requires more than just failed efforts at U.N. approval. The doctrine stipulates that member states have an affirmative duty to prevent crimes against humanity even when the international community fails to satisfy its obligation. In other words, member states are obliged to take action when the Security Council doesn’t.

This is perhaps the most controversial aspect of R2P. Military intervention outside an authorization by the U.N. arguably violates U.N. prohibitions against unauthorized use of force. Supporters of R2P argue, however, that R2P is rooted in other equally important U.N. doctrines, such as the Chapter VII authorization to use force in response to a threat to peace. And there’s a growing sentiment that R2P has emerged, by custom among nations, as an international law norm that forms an exception to the prohibitions against use of force.