Originating in theological discourse during the Middle Ages, the so-called Just War Doctrine was incorporated into positive international law. Drawing on still more ancient practices of political communities, the Just War Doctrine called upon participants in war to carry on combat with due respect for moral principles, including an overriding obligation to confine military attack to military targets, thereby avoiding any direct injury to civilians.
The application of the principles of the Just War Doctrine were left to each sovereign ruler, and amounted to an appeal to conscience. There was no higher authority aside from the questionable claim by the Roman Church that lost whatever overall validity it might have once possessed as a result of the fragmenting of Christendom due to the Reformation of the sixteenth century. In a sense, the Just War Doctrine was an instance of religious morality being converted into an applied ethics for international relations. It was borrowed and incorporated into international law at the inception of the state system.