This volume presents the first full English translation of four key texts from the dispute between Juan Gines de Sepulveda and Bartolome de las Casas regarding the justice of Spain’s invasion of the Americas, culminating in their famous debate in Valladolid in 1550-51. An impassioned defence of the invasion, Sepulveda’s Democrates secundus (composed around 1544) amplified the controversy within Spain about the justice of its activities in the Americas. When Las Casas schemed to block publication of Sepulveda’s manuscript, Sepulveda wrote an Apologia (1550) in its defence. Tensions were so high 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, Sepulveda and Las Casas debated bitterly. Las Casas subsequently printed a composite record of the Valladolid deliberations (Aqui se contiene una disputa o controversia, 1552). Sepulveda retaliated by penning a furious response (Proposiciones temerarias y de mala doctrina, around 1553-54) and strove to have Las Casas’ text banned by the Inquisition. The debate between Sepulveda 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. With a detailed scholarly introduction that elucidates the complex story of these four controversial texts and reflects on the impacts of Sepulveda’s ideas, which continue to be felt in the theories and practices of war today, this book is a must-read for all those interested in the fields of history, political science, international relations, and colonial studies.