The culmination of the clash between two conceptions of infidel dominium and world order expressed itself most dramatically at the Valladolid junta of the Spanish imperial court. This debate put on perspicuous display two opposed ethics of evangelization embedded in the theology and canon law of the Latin West: an apostolic method of peaceful preaching and a coercive method of missionary war. Las Casas and Sepúlveda, respectively, took up these two approaches with startling results. Drawing on biblical and theological legal sources, both deployed natural law and the law of nations to advance their political arguments concerning Christian-infidel relations, thus signaling the ambivalent ideological tone of international relations in the West. Whereas Sepúlveda defended the justice of Spanish imperial dispossession of native peoples through war, Las Casas radically accounted for Amerindian claims of just war against European aggressors. By turning to Valladolid in depth, the unique scholastic theological and juristic contributions of Las Casas and the Spanish Dominicans appear with greater salience for international legal thought. Additionally, the understated ideological vestiges of Sepúlveda’s Spanish imperial humanism for early modern and modern European expansion also come into sharp relief.