The practice of humanitarian military action has changed markedly in the 21st century when compared with the 1990s. This essay explores three broad trends that have shaped this evolution. First, the UN has adopted the protection of civilians as a central element of its agenda and as a guiding principle for reforming its peace operations and its responses to atrocities such as genocide and ethnic cleansing. Second, major powers have played a central role as belligerents or patrons of belligerents in many of the worst conflicts of the last two decades. And third, the wealthy Western states with the greatest resources and military capabilities for ambitious humanitarian operations have substantially reduced their direct contributions to these missions. Together, these developments have shifted the balance of responsibility and effort for humanitarian military operations toward the UN and developing countries; constrained the ambitions of these missions; limited what they can accomplish and contributed to gaps between the expectations they create and the protection they are able to deliver; and discouraged meaningful action in response to many of the century’s most devastating conflicts.