Two seemingly unrelated crises implicating the law of war and the responsibility to protect civilians have arisen in recent years. In 2013, the United States considered military intervention without U.N. Security Council preapproval in Syria after discovering that the government had exterminated its own people with chemical agents. In 2014, Russia sent troops into Crimea, a part of Ukraine, to protect ethnic Russians that Russia claimed were in danger after a political coup in the country. In both cases, the military acts contemplated or undertaken were of dubious legality, albeit under different rubrics. This Article aims to show how analysis of the lawfulness of military intervention in Syria and Crimea is illuminated by recognizing that both are subspecies of the same problem and may thus be seen as one customary doctrine of international law governing the grounds for war. By custom, a sovereign state may use force in another unconsenting sovereign state without U.N. Security Council authorization or a selfdefense justification to protect civilians facing imminent risk of group extermination—a threshold that was arguably met in Syria but seemingly not in Crimea. The right to use armed force in such instances is further constrained by the proportionality and exhaustion-of-other means requirements that generally apply to the law of war. This customary legal right to use force was traditionally limited to protecting the lives of the intervening state’s own civilians for two related reasons. First, the bedrock principle of exclusive sovereignty shielded a target state’s treatment of its civilians within its borders. Second, there was a consensus that international law did not permit the use of armed force to enforce the right against death of civilians in another country absent the nexus of nationality to the victims. However, in the past dozen years, both principles have been fatally undermined by the norm of the “responsibility to protect” civilians, which pierces the veil of sovereignty for states that harm or fail to protect their own peoples. Consequently, the present customary international law of war can reasonably be construed as extending the ancient unilateral civilian-protection use-of-force easement to the use of force to protect all civilians facing state-sponsored mass killings, regardless of nationality. The life-saving easement on sovereign territory logically covers only cases where civilians are facing group death—genocide, massacre killings, or lethal use of atomic, biological, or chemical weapons— initiated by the host state which is violating its duty to protect the people within its territory. U.N. Security Council authorization or a self-defense justification is still required for military interventions in response to other mass atrocities such as ethnic cleansing, war crimes, state-initiated individual killings, non-lethal crimes against humanity (for example, systemic torture), or the possession (or non-lethal use) of weapons of mass destruction, and to humanitarian crises where deaths are not caused by the state or its agents. Of course, any intervening state’s decision to use armed force to protect its own or foreign civilians in an unconsenting state is ultimately a matter of its own domestic law and policy choice, but international law does not prohibit such a choice in the face of state-initiated mass killings.