President Obama declared that Bashar Al-Assad's use of chemical weapons on several thousand Syrian civilians (not the slaying and displacement of millions otherwise) was a breach of international norms so severe that use of force was justified. After Obama asked for congressional and UN Security Council backing, Congress and the British Parliament demurred. Then Assad's patron, Russia, brokered a deal to have the chemical weapons removed.
The message to Assad: America and the world won't use force to punish chemical weapons, and worse yet, if you want to get off untouched for mass atrocities committed with more ordinary weaponry, acquire and use some weapons of mass destruction as a bargaining chip for retaining power.
As UN Secretary General, norm entrepreneur Kofi Annan helped uncork the humanitarian doctrine of "Responsibility to Protect" ("R2P" in social media-ese).
In the wake of Bosnia and Rwanda, he asked Canada to host a commission on atrocities that, steered by its Australian co-chair Gareth Evans, coined that phrase in its 2001 report. Then in 2005, heads of state formallyadopted R2P at the World Summit marking the 60th anniversary UN General Assembly.
R2P is said to consist of three pillars.