The tradition in moral and political philosophy that attempts to specify when the resort to war is morally permissible (jus ad bellum) and how war, once begun, must be conducted (jus in bello) — developed through Augustine, Aquinas, Grotius, and Vitoria, and now institutionalized in international humanitarian law, while remaining contested by both pacifists who reject its permissive conclusions and realists who reject its moral constraints.
Just war theory promises something that should be impossible: a set of moral criteria rigorous enough to distinguish legitimate from illegitimate wars, applied by the very parties who are about to fight them. Every state that has ever gone to war has claimed the justice of its cause. The theory was not designed to prevent this — it was designed to discipline it, to specify what genuine justification requires and expose the cases where it is merely claimed. Whether it has succeeded is the central question. The theory exists because the alternatives — that all wars are equally unjust, or that wars require no justification at all — are both morally and politically untenable.