There is a growing body of research focusing on the degree to which humanitarian intervention is allowed and legitimised by international society. This research, conducted by constructivist and English school scholars, can be described as an investigation into the nature and strength of a norm permitting humanitarian intervention. However, scholarship has tended to neglect a parallel but discrete norm of humanitarian intervention. It is a contention of this article that ideas and beliefs shared by liberal members of international society not only permit intervention but prescribe it in certain circumstances, and this has been largely overlooked in the literature. The objective of this article is to give an account of the impact of the norm prescribing humanitarian intervention on the response of the United States government to the Rwandan genocide.