Saint Thomas Aquinas, O.P.
In the second place, there must be a just cause; that is to say, those attacked must have, by a fault, deserved to be attacked. This is what makes St. Augustine say in Book VI, Question 16, of Questions on Joshua: "Just wars are usually defined as those which avenge injuries, when the nation or city against which warlike action is to be directed has neglected either to punish wrongs committed by its own citizens or to restore what has been unjustly taken by it. Further, that kind of war is undoubtedly just which God Himself ordains."
In the third place, it is necessary that the intention of those who fight should be right; that is to say, that they propose to themselves a good to be effected or an evil to be avoided. This is what made St. Augustine say in the book De Verbis Domini: "With the true servants of God wars themselves are pacific, not being undertaken through cupidity or cruelty, but through the love of peace, with the object of repressing the wicked and encouraging the good." Consequently, it may happen that, although the war has been declared by the legitimate authority and for a just cause it may nevertheless be rendered illicit by the perversity of the intention of him who makes it. "For," says St. Augustine (Contra Faustum, I, XXII, Chap. 74), "what is blamed in war? Is it the death of those who must die sooner or later, but who give up their lives to bring peace by overcoming guilty men? To blame this is the cry of cowards, not of religious people. The desire for harming, the cruelty of avenging, an unruly and implacable animosity, the rage of rebellion, the lust of domination and the like-these are the things which are to be blamed in war."
To the second argument (viz. that war is a sin, as being "contrary to a divine precept") it must be replied that these precepts, as St. Augustine says (De Serm. Domini in monte, I, 34), ought always to be observed in relation to the disposition of the soul; that is to say, that man ought always to be ready, if necessary, not to resist or not to defend himself. But sometimes we must act otherwise for the common good, and even for the good of those against whom we fight. This it is that causes St. Augustine to say, in the Fifth Epistle Ad Marcellinum: "There are many things that must be done against the will of those whom one ought to correct with a beneficent severity."
To the third argument the reply is, that those who wage wars justly have peace as the object of their intention, and so they are not opposed to peace, but only to that evil peace which the Lord did not come on earth to bring (St. Matthew x: 34). Hence Augustine says (Ep. ad Bonifacium, CLXXXIX) : "For peace is not sought in order to the kindling of war, but war is waged in order that peace may be obtained. Therefore, even in waging war, cherish the spirit of the peacemaker, that, by conquering those whom you attack, you may lead them back to the advantages of peace . . . ."
To the fourth argument the reply is that manly exercises in warlike feats of arms are not all forbidden but those which are inordinate and perilous, and end in slaying or plundering. In olden times warlike exercises presented no such danger and hence they were called exercises of arms or bloodless wars.
Aquinas, St. Thomas. "The Just War." In The Summa Theologica. Great Books of the Western World vol. 20 (Chicago, Illinois: Encyclopedia Britannica, Inc., 1952).