Francisco de Vitoria (1483–1546) was among the preeminent theologians of his day and his two texts, De Indis and De Iure Belli, mark the start of a vitally important transition in the Christian just war tradition as it exited a medieval social imaginary and entered a modern one. Not only are there glimpses of early modernist just war thought and a revolutionary reframing of natural law thinking in these texts, but they find their starting point in one of the most acute questions in all of just war thinking: how to understand and engage an “other,” most notably indigenous persons in the Americas and West Indies. Vitoria’s surprisingly progressive answers to this question moved the tradition forward, powering its increasing political scope and moral significance. They also shaped failures – most notably in funding modern notions of race and the rise of chattel slavery while also shaping early modern conceptions of property and ownership – and caused suffering for which the tradition is at least partly accountable and lacunae that it must now overcome as it moves into the environmental age.