This article traces the rise of humanitarian interventionist ideas in the US from 1991 to 2003. Until 1997, humanitarian intervention was a relatively limited affair, conceived ad hoc more than systematically, prioritized below multilateralism, aiming to relieve suffering without transforming foreign polities. For this reason, US leaders and citizens scarcely contemplated armed intervention in the Rwandan genocide of 1994: the US 'duty to stop genocide' was a norm still under development. It flourished only in the late 1990s, when humanitarian interventionism, like neoconservatism, became popular in the US establishment and enthusiastic in urging military invasion to remake societies. Now inaction in Rwanda looked outrageous. Stopping the genocide seemed, in retrospect, easily achieved by 5,000 troops, a projection that ignored serious obstacles. On the whole, humanitarian interventionists tended to understate difficulties of halting ethnic conflict, ignore challenges of postconflict reconstruction, discount constraints imposed by public opinion, and override multilateral procedures. These assumptions primed politicians and the public to regard the Iraq war of 2003 as virtuous at best and unworthy of strenuous dissent at worst. The normative commitment to stop mass killing outstripped US or international capabilities—a formula for dashed hopes and dangerous deployments that lives on in the 'responsibility to protect'.