The theory of just war in medieval canon law and theology has attracted to it a large body of scholarship, and is recognized as an important foundation for Western approaches to the study of ethics in war. By contrast, the tradition on war in medieval Roman law has not received much attention, although it developed doctrines that are distinct from those in canon law and theology. The oversight is notable because medieval Roman law on war influenced subsequent tradition, forming with canon law the essential basis for early modern legal thought on war and peace. While the main canonistic contributions to legal theory on war came in the twelfth and thirteenth centuries, Roman jurists added new opinion in the fourteenth and fifteenth centuries, which can be related to the political life of Italy and to the growth of the independent cities. By the fourteenth century, Roman lawyers (or civilians) often considered licit war from a secular and pragmatic perspective, and associated a right of war with sovereignty. Here, I would like to trace the development of this theory, from roughly 1250 to 1450, and particularly a view that sovereigns licitly judged the justice of their own causes, as a remedy for a lack of superior authority.