This volume presents translations of four key texts from the dispute between Juan Ginés de Sepúlveda and Bartolomé de las Casas regarding the justice of Spain’s invasion of the Americas, culminating in their famous debate in Valladolid in 1550–51. Sepúlveda’s Democrates secundus (composed around 1544), an impassioned defence of the invasion, amplified the controversy within Spain about the justice of its activities in the Americas. When Las Casas schemed to block publication of Sepúlveda’s manuscript, Sepúlveda wrote an Apologia (1550) in its defence. Tensions were so heightened that Emperor Charles V called a temporary halt to undertakings in the Americas and convoked a meeting of theologians and jurists in Valladolid to address the matter. Here, Sepúlveda and Las Casas debated bitterly. Las Casas subsequently printed a composite record of the Valladolid deliberations (Aquí se contiene una disputa o controversia, 1552). Sepúlveda retaliated by penning a furious response (Proposiciones temerarias y de mala doctrina, around 1553–54) and strove to have Las Casas’s text banned by the Inquisition. Complete English translations of these texts are here published for the first time. The debate between Sepúlveda and Las Casas was a pivotal moment in the history of international legal thought. They argued over fundamental matters of empire and colonial rule; natural law and cultural difference; the jurisdiction of the Church, responsibilities of Christian rulers, and rights of infidel peoples; the just reasons for war and grounds for resistance; and the right to punish idolatry, protect innocents from tyranny, and subjugate unbelievers for the purpose of spreading the Christian faith.