The theoretical reflection formulated in the neo-scholastic School of Salamanca has been regarded as the source of the contemporary concept of international relations: going beyond the paradigm of the time – determined, on the one hand, by the idea of the holy war and, on the other, by the doctrine of the just war – the Salamanca scholars laid down foundations for the edifice of modern international law. In this article the author argues that similar ideas preceding the reflection of the school of Salamanca, and often going even further, had appeared a hundred years earlier in the Cracow Academia in the context of a dispute between the Kingdom of Poland and the Teutonic Order. The culmination of this dispute was the Council of Constance. The Polish argument after the battle of Grunwald was based on law, and its unique feature was the return to the universalistic (Augustinian) nature of the just war. What is the most important element in Augustinian theology of war is Christianity's universalism and recognition of moral equality of those engaged in military operations.