Those who treat of such matter usually enumerate four conditions of a just war, lawful authority, a just cause, a good intention, and a suitable method. But each must be discussed by itself.
The first condition, then, is lawful authority. For St. Augustine says, “The proper order among human beings, adapted to peace, demands this, that there be authority for undertaking a war and deliberation on the part of the leader, but the soldiers owe to common peace and safety the duty of carrying out orders.” 247 And reason proves this; for private citizens, and those who have a superior, if they are injured by any one, can have recourse to the superior, and seek judgment from him. But if rulers suffer anything at the hands of another ruler, they have no common tribunal before which they may accuse the aggressor, and therefore it is lawful for them to oppose public wrongs by war.
Moreover, this authority for declaring war resides, according to common opinion, in all rulers, and in nations, who in temporal affairs have no superior, such as are all kings, likewise the Republic of Venice and similar States, and likewise some Dukes and Counts who are subject to no one in secular matters; not, however, those Dukes and Counts who are immediately subject to kings, for those who are subject to others are not in themselves heads of the State, but rather members. Note, nevertheless, that this authority is not requisite for defensive war, but only for offensive; for it is lawful for everyone to defend himself, whether he be a ruler or a private citizen, but to declare war, or to invade the territory of an enemy, is lawful only for the supreme head.
The second condition is a just cause; for war cannot be declared for any offense at all, but only for the purpose of warding off an injury. Thus St. Augustine says: “Just wars are usually defined as those in which injuries are avenged, when any nation or city which is to be attacked in war has either to satisfy for what has been wickedly done by it, or to return what has been unjustly taken.” 248
The reason for this is that a ruler is the judge of his own subjects only, therefore he cannot punish any crimes committed by the subjects of others, but only those which happen to the harm of his own subjects; for even if he is not the ordinary judge of others, yet he is the defender of his own people, and by reason of this obligation it comes to pass that he is also to a certain extent the judge of those who do harm to his people, so that he can punish them with death.
Indeed, it should be observed that the cause of war should be neither trivial nor doubtful, but weighty and certain, lest, perchance, the war bring about more harm than the hoped-for-good, hence if there is any doubt a distinction must be made between the ruler and the soldiers, for the ruler himself sins, without doubt; for war is an act of retributive justice, but it is unjust to punish any one for a cause not yet proved; but the soldiers do not sin unless it is plainly evident that the war is unlawful, for subjects ought to obey their superior, nor should they criticize his commands, but they should rather suppose that their ruler has a good reason, unless they clearly know the contrary; just as when the offense of some particular individual is doubtful, the judge who condemns him sins, but not the executioner who carries out the sentence of death imposed on the condemned; for the executioner is not bound to criticize the sentence of the judge. So teaches Pope Boniface. “Whoever,” says he, “carries out the orders of a judge seems to have done no wrong, since he is under the necessity of obeying.” 249 And St. Augustine says, “Thus it is that a just man, if perchance he is fighting under a wicked king, can justly fight under his orders, preserving the order of civic peace, provided he is certain that what he is commanded is not against the command of God, or even when it happens that he is not certain, so that while perhaps the wickedness of the command makes the king a criminal, the duty of obeying proves the soldier to be guiltless.” 250
Note, however, that this is to be understood of soldiers who are bound to a prince who is waging war, namely, that they are his subjects, and also of those who receive a regular salary from him even in times of peace, but not of those who come from another place when war is to be carried on; for these are not obliged to fight, nor can they take part in the war with a safe conscience unless they know the war is just; but those who give no thought to the matter, but are ready to fight whether the war is just or not, provided they are paid, are on the road to damnation.
The third condition is a good intention. For, since the end of war is peace and public tranquillity, it is not lawful to undertake war for any other end, hence those sin seriously, whether rulers or soldiers, who begin a war either to injure someone, or to extend their empire, or to show warlike prowess, or for any other cause than the common good, even if lawful authority and a just cause are not lacking. So St. Augustine, in the Epistle to Count Boniface, says “The will should esteem peace, necessity only should bring about war, that God may deliver us from the necessity of war and preserve us in peace; for peace is not sought in order that war may be waged, but war is waged that peace may be attained. Be ye, therefore, desirous of peace even in war, so that those whom you are fighting may be brought by conquest to the unity of peace.” 251 And, “Desire of injuring, cruelty in avenging, an unpacified and implacable spirit, fierceness in renewing war, lust for power, and any similar faults, these are the things which are rightly condemned in war.” 252
Two things, however, should be noted. First, since war is a sort of means to peace, but very hard and dangerous, therefore war should not be begun in haste, when there is cause, but peace should first be sought by some easier means, namely, by peacefully seeking the reparation due from the enemy. “If at any time thou come to fight against a city, thou shalt first offer it peace, etc.” 253 And St. Augustine says, “The will should esteem peace, necessity only should bring about war.” 254
But some one may ask, if the enemy at first is unwilling to make satisfaction, yet almost immediately after, when the war has been begun, seeks peace and offers reparation, whether in such a case his opponent is bound to give up war. Cajetan, under the heading Bellum, says that he is not bound to give it up when the war has already been begun, although before it was begun he was bound to accept satisfaction. But (with due deference to any better opinion) it seems that we ought to say that he who has a just cause is never bound in justice to accept satisfaction, either before the beginning of the war of after; yet by reason of charity he is bound in both cases, unless something to the contrary intervenes. The reason for the first is that a ruler having just cause for war bears the character of judge with regard to the other ruler who has done him an injury, but a judge is not bound in justice to pardon a criminal from the death penalty, even if he offers satisfaction, although, if he is the supreme judge, he can pardon him out of mercy. For example, a king is not bound to spare the life of a thief who makes restitution, although he may do so through mercy. The reason for the latter is that war is a most severe infliction, by which not only he who has offended is punished, but incidentally many innocent persons are also involved. Therefore, Christian charity seems always to exact this, that the war should end when he who has done the injury offers the satisfaction due, unless by chance something has accidentally intervened, that is, unless the enemy against whom war is being waged be such that it is expedient for the common good either that he be made subject to another or that he be utterly destroyed, and such were the Amalechites whom God ordered to be utterly wiped out. 255
Secondly, it should be noted that this third condition differs from the two preceding, because if these are absent they make the war unjust, but if this last is absent it makes the war evil, but not, strictly speaking, unjust. For whosoever begins a war without authority or without a just cause, sins not only against charity, but also against justice, and he is not so much a soldier as a robber; but he who has authority and a just cause, and yet makes war from a love of revenge, or the desire of increasing his domain, or for any other evil motive, does not act against justice, but only against charity, and is not a robber, but a wicked soldier. From which it follows that when this third condition alone is lacking, neither soldiers nor kings are bound to any restitution, but only to repentance; but when the first or the second is lacking, all are bound to repair the damage inflicted, unless they are excused by reason of invincible ignorance; for just as gross and culpable ignorance does not excuse from sin, so neither does it excuse from restitution, as appears in the last chapter on injuries and the loss inflicted. But he who acts in invincible ignorance is not bound to restitution while he so acts, but when he recognizes that the war is unjust he is bound to restitution, not for injury inflicted during the war, but he is if through that war he has acquired anything that does not belong to him; and if he has taken no money, but yet has become richer by the sale of goods, he is bound to make restitution to the extent to which he has enriched himself; for he cannot retain the property of another, even if by reason of ignorance he has acquired it without sin, but restoration should be made either to the original owner, if he is known, or to the poor.
The fourth condition is the suitable manner, which consists chiefly in this, that no innocent person be harmed, as John the Baptist explains, “Do violence to no man; neither calumniate any man; and be content with your pay.” 256 By these words he forbids the injuries which soldiers usually inflict on the innocent, either by violence or by trickery, whether to person or to property. When he says, “Do violence to no man,” he forbids the injury which open violence inflicts on one’s person, as when soldiers kill peasants who do not readily obey. When he says, “neither calumniate any,” he forbids the injury done by fraud and calumny, as when soldiers say that a certain man is a traitor or an enemy although they know that the contrary is true, and on this charge they either rob him themselves, or they kill him, or they bring him before the ruler or the prefects. And when he adds, “and be content with your pay,” he forbids the injury done not to the person, but to his property, as when soldiers rob and plunder wherever they can, or even exact and extort from those from whom they should not.
But it must be observed that there are three classes of persons on whom soldiers cannot inflict injury, according to the rule of John the Baptist. The first class is that of all those who are not enemies of the State; by reason of which soldiers cannot be excused who inflict injury on citizens, or friendly peasants on whom they are quartered, or through whose property they are passing, and they are not excused if they say that their salary has not been paid to them; for the goods of private citizens are not therefore due them, nor should they inflict punishment on a citizen or a peasant, if the king or the ruler sins by not paying the wages to the soldiers, unless by chance, for just cause, the men of some certain place have been condemned to this punishment, namely, that they must maintain the soldiers, which, however, happens more rarely than otherwise.
The second class is that of those who, even if they are in some way enemies of the State, yet are excused in the canon Innovamus, on truce and peace, where it is written thus: “We decree that priests, monks, those in religious houses, pilgrims, merchants, farmers either going to or returning from market, or employed in agriculture, and the beasts with which they plow or carry seed to the field, are to enjoy fitting security.” Here by the name merchants are not meant those who live in the country of the enemy, and form a certain part of the State, but only those who are passing through or who are coming for the market-day, who are not members of the State.
The third class is that of those unfit for war, such as are children, old men, and women; for such, even if they can be seized and robbed, since they are part of the State, yet they cannot justly be killed, unless perchance they are slain unintentionally or accidentally; as when a soldier casts a javelin against a battalion of the enemy, and by chance a child, or a woman, or even a priest, is killed, the soldier does not sin, but he sins when he kills intentionally, and can, if he wishes, refrain from killing; for both natural reason teaches this, and God has also commanded this to the Jews, 257 namely, that they should spare the children and the women, and Theodosius was seriously reproved by St. Ambrose because, when he wished to punish the Thessalonians, he ordered all those whom he met to be killed without discrimination, as Theodoret relates. 258 But if Moses sometimes ordered even women and children to be killed, 259 this is not therefore lawful for our soldiers; for Moses clearly knew from the Revelation of God, that God, to Whom no man can say, “Why dost Thou thus?” so willed it.
247 Against Faustus, Bk. XXII., ch. 75.
248 Q. 10 on Josue.
249 Sext on legal rules, Rule 25.
250 Against Faustus, Bk. XXII., ch. 75.
251 Epis. 205 or 207.
252 Against Faustus, Bk. XXII.
253 Deut. XXII., 10.
254 Epis. 207 to Boniface.
255 Deut. XX.
256 Luke III., 14.
257 Deut. XX.
258 History, Bk. V., ch. 17 and 18.
259 Deut. II. and III. and elsewhere.