This chapter examines the roots of Responsibility to protect (RtoP) in international law and international ethics. It explores how RtoP evolved out of the crisis in Kosovo, and discusses its policy significance today in the controversial case of Libya. Domestically inflicted war crimes and crimes against humanity in non-international, civil-war conflicts are parts of international law outlawed by custom and treaties, including the Rome Statute defining the jurisdiction of the International Criminal Court. Humanitarian intervention is differentially rooted in international ethics, central and fundamental to Liberalism, marginal and instrumental to Realism, and relevant but tangential to Marxism. It is conflicted in international law, required to stop genocide, but rejected by Charter law. The UN-authorized and NATO-led intervention in Libya in March 2011 was the just-war doctrine's most important test case. The intervention in Libya joined legality (Security Council approval) to legitimacy (the cause of protecting civilians).