Most of the major theoretical traditions in international relations offer little advice on how costly international moral action could be accomplished. The main exception is the constructivist approach that focuses on the spread of cosmopolitan ethical beliefs through transnational interaction. While the logic of this theory does not imply any limit on the scale of goals that might be achieved, most constructivist empirical work so far has focused on relatively inexpensive moral efforts, such as food aid, and so may not identify the conditions under which states will take on much more costly moral projects. In this article, we test the constructivist theory of moral action against the record of the most costly international moral action in modern history: Britain's sixty-year effort to suppress the Atlantic slave trade from 1807 to 1867. We find that the willingness of British abolitionists to accept high costs was driven less by a cosmopolitan commitment to a moral community of all people than by parochial religious imperatives to impose their moral vision on others and, especially, to reform their domestic society. Transnational influences also had no important effect. Rather, the abolitionists' success in getting the British state to enact their program was determined mainly by opportunities provided by the fragile balance of power m British domestic politics. Although testing in more cases is needed, these findings suggest that better explanations of international moral action might be provided by a type of domestic coalition politics model based on what we call “saintly logrolls.”