In Renaissance Europe, war and the use of force were regular phenomena and likewise subject of common rules. The theory of ‘just war’, dating back to ancient times, was further developed by legal scholars, and all belligerents claimed to have a just cause, often explained in printed pamphlets. International law consisted of theory and practice, and thus, they should be considered in a mutual context. The focus was still on the question of who had a right to wage war, i.e. on the jus ad bellum, and barely on regulations of warfare or on a containment of war. International law in Renaissance Europe rooted in its very society, in its rules and values. Legal debates and war justifications consolidated the Christian European community, which even in war times did not break apart, even if it was contradictory to the principle of sovereignty and the idea that a sovereign owes no justification to anyone. Moreover, scholars and belligerents argued with natural law and insisted in the universality of international law, although it was in fact basically European. Thus, in Renaissance Europe well-established traditions existed for how to handle war, but they were more and more challenged by the idea of sovereignty, as well as by the European expansion and by global interaction.