In the wake of the United States Civil War, a transatlantic network of former abolitionists launched a new movement that made sex -- hitherto the jurisdiction of the family and the local community -- into an international political issue, intimately linked to imperialism, militarism, immigration, labor, temperance, and women's rights. My dissertation, "Purifying the World: Americans and International Sexual Reform, 1865-1933," examines American reformers who saw sexuality as the key international humanitarian and political issue of their day. My project tracks this reform movement from its beginning -- with the work of American abolitionists and missionaries who turned their attention to stateregulated prostitution in the British Empire after the Civil War -- to its denouement in the activities of interwar Americans who traveled the globe investigating the "traffic in women" for the League of Nations. For over a halfcentury, American reformers participated in pitched international debates about how to address sexual wrongs. As they did so, they wove together religious, medical, and legal discourses in ways that made sexual matters the provenance of international politics.