The United States has historically been inconsistent and ambivalent about the responsibility to protect. Part 1 of the article sets out a theoretical framework for understanding how the United States aligns itself with the responsibility to protect; it does so by initially using the idea of norm localisation, which reveals important convergences and tensions between the international norm and the localised variant that we call ‘genocide and mass atrocity prevention/protection’. Part 2 looks at the impact of this norm innovation in relation to the position that the United States government adopted on Libya – suggesting that it played a critical leadership role in the crisis and in doing so took risks with its international reputation while knowing that there was little prospect that this action would be warmly greeted by Congress or domestic public opinion.