Contrary to what has been alleged, the moral equivalence of combatants (MEC) is not a doctrine that was expressly developed by the traditional theorists of just war. Working from the axiom that just cause is unilateral, they did not embrace a conception of public war that included MEC. Indeed, MEC was introduced in the early fifteenth century as a challenge to the then reigning just war paradigm. It does not follow, however, that the distinction between private and public war had no place in the traditional teaching. Thomas Aquinas and his successors did not analyse just war by extrapolation from the related idea of self-defense. Rather, they likened just war to a legal proceeding that could solely be undertaken by persons possessed of legitimate authority. For this reason, just war was first and foremost public war. Private war was deemed ‘war’ only in a secondary and reduced sense of the term. It was accordingly understood that public war should be waged and its morality judged by reference to a set of norms that are not directly reducible to those governing private self (and other)-defense.