Objection 1. It would seem that it is always sinful to wage war. Because punishment is not inflicted except for sin. Now those who wage war are threatened by Our Lord with punishment, according to Matthew 26:52: "All that take the sword shall perish with the sword." Therefore all wars are unlawful.
Objection 2. Further, whatever is contrary to a Divine precept is a sin. But war is contrary to a Divine precept, for it is written (Matthew 5:39): "But I say to you not to resist evil"; and (Romans 12:19): "Not revenging yourselves, my dearly beloved, but give place unto wrath." Therefore war is always sinful.
Objection 3. Further, nothing, except sin, is contrary to an act of virtue.
But war is contrary to peace. Therefore war is always a sin.
Objection 4. Further, the exercise of a lawful thing is itself lawful, as is evident in scientific exercises. But warlike exercises which take place in tournaments are forbidden by the Church, since those who are slain in these trials are deprived of ecclesiastical burial. Therefore it seems that war is a sin in itself.
On the contrary, Augustine says in a sermon on the son of the centurion [Ep. ad Marcel. cxxxviii]: "If the Christian Religion forbade war altogether, those who sought salutary advice in the Gospel would rather have been counselled to cast aside their arms, and to give up soldiering altogether. On the contrary, they were told: 'Do violence to no man... and be content with your pay' [Luke 3:14. If he commanded them to be content with their pay, he did not forbid soldiering."
I answer that, In order for a war to be just, three things are necessary. First, the authority of the sovereign by whose command the war is to be waged. For it is not the business of a private individual to declare war, because he can seek for redress of his rights from the tribunal of his superior. Moreover it is not the business of a private individual to summon together the people, which has to be done in wartime. And as the care of the common weal is committed to those who are in authority, it is their business to watch over the common weal of the city, kingdom or province subject to them. And just as it is lawful for them to have recourse to the sword in defending that common weal against internal disturbances, when they punish evil-doers, according to the words of the Apostle (Romans 13:4): "He beareth not the sword in vain: for he is God's minister, an avenger to execute wrath upon him that doth evil"; so too, it is their business to have recourse to the sword of war in defending the common weal against external enemies. Hence it is said to those who are in authority (Psalm 81:4): "Rescue the poor: and deliver the needy out of the hand of the sinner"; and for this reason Augustine says (Contra Faust. xxii, 75): "The natural order conducive to peace among mortals demands that the power to declare and counsel war should be in the hands of those who hold the supreme authority."
Secondly, a just cause is required, namely that those who are attacked, should be attacked because they deserve it on account of some fault. Wherefore Augustine says (QQ. in Hept., qu. x, super Jos.): "A just war is wont to be described as one that avenges wrongs, when a nation or state has to be punished, for refusing to make amends for the wrongs inflicted by its subjects, or to restore what it has seized unjustly."
Thirdly, it is necessary that the belligerents should have a rightful intention, so that they intend the advancement of good, or the avoidance of evil. Hence Augustine says (De Verb. Dom. [The words quoted are to be found not in St. Augustine's works, but Can. Apud. Caus. xxiii, qu. 1): "True religion looks upon as peaceful those wars that are waged not for motives of aggrandizement, or cruelty, but with the object of securing peace, of punishing evil-doers, and of uplifting the good." For it may happen that the war is declared by the legitimate authority, and for a just cause, and yet be rendered unlawful through a wicked intention. Hence Augustine says (Contra Faust. xxii, 74): "The passion for inflicting harm, the cruel thirst for vengeance, an unpacific and relentless spirit, the fever of revolt, the lust of power, and such like things, all these are rightly condemned in war."
Reply to Objection 1. As Augustine says (Contra Faust. xxii, 70): "To take the sword is to arm oneself in order to take the life of anyone, without the command or permission of superior or lawful authority." On the other hand, to have recourse to the sword (as a private person) by the authority of the sovereign or judge, or (as a public person) through zeal for justice, and by the authority, so to speak, of God, is not to "take the sword," but to use it as commissioned by another, wherefore it does not deserve punishment. And yet even those who make sinful use of the sword are not always slain with the sword, yet they always perish with their own sword, because, unless they repent, they are punished eternally for their sinful use of the sword.
Reply to Objection 2. Such like precepts, as Augustine observes (De Serm. Dom. in Monte i, 19), should always be borne in readiness of mind, so that we be ready to obey them, and, if necessary, to refrain from resistance or self-defense. Nevertheless it is necessary sometimes for a man to act otherwise for the common good, or for the good of those with whom he is fighting. Hence Augustine says (Ep. ad Marcellin. cxxxviii): "Those whom we have to punish with a kindly severity, it is necessary to handle in many ways against their will. For when we are stripping a man of the lawlessness of sin, it is good for him to be vanquished, since nothing is more hopeless than the happiness of sinners, whence arises a guilty impunity, and an evil will, like an internal enemy."
Reply to Objection 3. Those who wage war justly aim at peace, and so they are not opposed to peace, except to the evil peace, which Our Lord "came not to send upon earth" (Matthew 10:34). Hence Augustine says (Ep. ad Bonif. clxxxix): "We do not seek peace in order to be at war, but we go to war that we may have peace. Be peaceful, therefore, in warring, so that you may vanquish those whom you war against, and bring them to the prosperity of peace."
Reply to Objection 4. Manly exercises in warlike feats of arms are not all forbidden, but those which are inordinate and perilous, and end in slaying or plundering. On olden times warlike exercises presented no such danger, and hence they were called "exercises of arms" or "bloodless wars," as Jerome states in an epistle [Reference incorrect: cf. Veget., De Re Milit. i].
Source: Summa Theologica (New Advent)