War, in the Catholic view, can be a good—not in and of itself, but by virtue of its end goal, so long as the means of it are within reasonable bounds. This view of violence has come to be known as the Just War Theory, being developed by the greats of Catholic thought, including St Augustine, and—of course—the Angelic Doctor, St Thomas Aquinas. But what exactly is a “just war,” and what examples throughout history can be identified as such?
Aquinas gives us the three definitive traits of the just war :
- The authority of he who declares the war is to be legitimate.
- The reason or cause for which the war is waged is to be just.
- The intention of the warriors is to be “the advancement of good, or the avoidance of evil.”
To the first point, as all men are by nature subject to authority (ref The Divine Autocrat)—be it the authority of God, the Church, or a just government—and as he who is in authority by definition has dominion over matters of which he is the authority of, he who declares war must have dominion over that which he orders to war, and over that which the war is regarding (or if he does not, it is under the authority and on the behalf of he who does), otherwise the declarer of war would be overstepping his dominion, doing business which is not his own—and this Plato tells us is unjust, as justice is “doing one’s own business” . Hence, if a war is just, it must be declared by he who has authority over those he calls to war, and over the matters of the war (or if not he himself, on behalf of he who does—eg a king may call to war his people over a moral matter deemed deserving of war by the Church, which has dominion over morality).
To the second point, St Augustine tells us that a just war is “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” . In general, as Aquinas puts it, “those who are attacked, should be attacked because they deserve it on account of some fault” . Hence, a just war is a war waged to end or correct some moral evil—be that evil an unjust government, the wrongful seizing of private property, or of some other kind. If war is declared out of a mere hatred of the enemy and/or a lust for power and riches, it is undoubtedly unjust.
And to the third point, which is that the intentions must be toward “the advancement of good, or the avoidance of evil” , a just war must be fought by those motivated in virtue. For Augustine tells us that “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” . Thus, the soldiers of war, in seeking to advance the good and avoid evil, can not themselves have immoral intentions—otherwise they will undoubtedly overstep the bounds of morality in combat, taking joy in violence, committing cruelty, and attacking the helpless and the noncombatants.
Now this third point covers more ground than the prior two—for they refer to the status of singular events related to the war, while the third remains ongoing for its entirety. If at any point the belligerents become disproportionately violent beyond what is necessary for victory, or attack non-combatants and other helpless people, or even if they begin to revel in some sick pleasure of killing, the war is—at least for as long as they are doing so—unjust and sinful. Moreover, the defiance of the hierarchy on illegitimate grounds too negates the just quality of a war. Hence, a war may be at some points just, but at others not.
Some examples of just wars include the following*:
- The First Crusade
- Declared by the Pope, who has authority over all men on the behalf of God
- Because of Islamic militants conquering other nations through unjust wars, committing a multitude of war crimes, and subsequently oppressing all non-Muslims
- Fought by warriors whose intentions were to save the oppressed Christians, and receive indulgences (a lessening of agony in Purgatory) for their pilgrimage to Jerusalem
- World War II
- Declared by the the leaders of the Allied nations, and supported by the Church
- Because of the Nazis committing mass genocide, conquering other nations through unjust wars, and committing a multitude of other war crimes
- Fought by warriors whose intentions were to save the victims of Nazi war crimes, to restore the rightful government of the conquered nations, and to put an end to Nazism
- The Spanish Conquest of the Aztec Empire (questionably)
- Declared by the Spanish government, and supported by the Spanish clergy
- Because of the Aztecs committing mass genocide, and ritual human sacrifice (but also for Spanish glory and wealth; hence this example is questionable)
- Fought by warriors whose intentions were to Christianize the natives, destroying the violent native religion, thus putting an end to ritual human sacrifice, and intentional genocide in native wars (unfortunately, the Spanish brought diseases with them which resulted in an unintentional—and thus morally irrelevant, but nevertheless bad—genocide)
To recapitulate, a just war is one which is declared by those legitimately in a position to do so, for the reason of correcting a moral wrong, and is fought by soldiers motivated to correct said moral wrong, who do not engage in that which is a moral evil per se—eg physically forcing the conquered to be baptized (again, this why the Spanish example is questionable, as what constitutes as a forceful conversion is up for debate, because the natives had no readily available alternative to their violent religion than Catholicism).
At certain points during each of these wars, unjust actions were taken; nonetheless, they remain just in general, but not in totality.
 Aquinas. ST II-II. Q4. A1. trans. Fr’s OP
 Plato. Rep IV, 433b, trans. Jowett
 Augustine. QQ in Hept. Q10. trans. Fr’s OP
 Augustine. Contra Faustum XXII.74, trans. Fr’s OP