The Romans can, with some justification, claim the credit for having introduced the most important legal and religious concept of war into the history of human thought. Thereby they made a very notable contribution to the historical development of international law. To the Romans we owe the idea of the ‘just and pious’ war which has for centuries characterized and influenced the thinking of Western Christendom about the place and practice of war. The idea is still with us today. Like the conception of natural law, the story of the ‘just and pious’ war is that of successive contents flowing though an ancient concept.

In the Rome of the kings it was the function of a special college of priests, the fetiales, to carry out the somewhat elaborate religious and legal procedures, the Jus fetiale, determining the declarations of war and peace between the Roman people and their enemies. The jus fetiale formed part of the Roman jus sacrum, and thereby imported a moral and legal element, over and above that of religion, into the formal procedures of establishing peaceful and hostile relations between Rome and other political communities. The task of the fetiales was to determine whether the duties owed to Rome by her neighbours had been violated, i.e., whether or not Rome had been wronged. These wrongs were well established and defined in Roman religiolegal thinking and formed part of the substantive content of the jus fetiale. These wrongs constituted the causes of ‘just and pious’ wars, to the Roman mind.