The great Aristotelian philosopher Thomas Aquinas discusses in the 2nd part of Summa Theologica the 3 conditions for a just war:
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.
About this Quotation:
As wars are being fought around us and in our name it is important that we be clear about the justness of these undertakings. The great Aristotelian philosopher St. Thomas Aquinas has three conditions which need to met before a war can be called “just”: does the prince who declares war have the correct authority to do so? does the war declaring nation have just cause to seek redress for an injury done to it? does the party declaring war have just intent in promoting good or avoiding evil? These are stringent conditions which have not been met very often, if ever, in the past. One recalls the long list of frivolous reasons for going to war which Thomas Gordon drew up.