St. Thomas Aquinas discusses the three conditions for a just war (1265-74)
The great Aristotelian philosopher St. Thomas Aquinas discusses in the 2nd part of Summa Theologica the three conditions for a just war:
War & Peace
The first thing is the authority of the prince by whose command the war is to be waged. It does not belong to a private person to start a war, for he can prosecute his claim in the court of his superior. In like manner the mustering of the people, that has to be done in wars, does not belong to a private person. But since the care of the commonwealth is entrusted to princes, to them belongs the protection of the common weal of the city, kingdom, or province subject to them. And as they lawfully defend it with the material sword against inward disturbances by punishing male-factors, so it belongs to them also to protect the commonwealth from enemies without by the sword of war.