Summa summae S. Thomae, Tomus III

War can be defined : «A clash by force and arms between two hostile armies.» By which it differs from a duel, which is a clash of one against one.

War is twofold : offensive, which is for obtaining satisfaction for an injury inflicted, and defensive, which is for repelling an injury being inflicted.

You will ask 1° whether war is licit.

[That war is always sin the Manichaeans once dogmatized, from St. Aug., lib. XXII contra Faustum, cap. 74; then the Wyclifites, as Waldensis testifies. But Luther proclaimed that to fight against the Turks was to contradict God willing through them to punish our sins. Erasmus contended that all war is forbidden Christians, against whom]

I answer. To make war is not per se evil, but, with three conditions posited, namely legitimate authority of declaring war, just cause and right intention, it is licit¹ .

Proof. from Scripture. In the ancient Law, God often commanded or approved war : Exod., XXII, 8 seqq. ; Num., XXI, 2 et 3 and XXV, 17 et 18 ; Deuter., VII, 1 seqq. ; I Reg., XV, 2 et 3 ; II Reg., VIII, 1 seqq. ; Josue, I, 2 seqq., VI, 2 seqq., VIII, 1 seqq., X, 8 seqq., and elsewhere often.

In the new Law² , Luc., III, 14, when soldiers asked what they should do that they might flee the coming wrath which John the Baptist threatened to sinners, the holy Precursor did not bid them desert military service, but to use it well : Do nothing violently, he says, nor accuse anyone falsely, and be content with your pay. By which words he tacitly approves war, as Augustine teaches, Epist. 138³ , to Marcellinus.

It is confirmed. from the usage of the Church, which in several councils decreed war for the recovery of the Holy Land.

Proof by reason. By the law of nature it is licit to repel force by force. Ergo defensive war is licit. As to offensive : A private man who has suffered injury or damage can seek satisfaction or restitution. Ergo likewise can the state. — I subsume. But sometimes satisfaction or restitution cannot otherwise be had than by arms, say if those who injure or damage, being required, refuse to satisfy or restitute, nor have they a superior who would or could compel them to satisfaction or restitution. Ergo.

I said : «With certain conditions posited, namely legitimate authority, just cause and right intention.»

The authority required for declaring an offensive war must be supreme⁴ , acknowledging no other superior in temporal matters, such as emperors, kings, certain dukes, independent states have, such as Venice, Switzerland, etc. 1° Because whoever has a superior can prosecute his right before him. But war cannot be undertaken except when there is no other way of prosecuting his right. 2° Offensive war is an act of supreme vindicative justice. Ergo it pertains to the supreme prince. But defensive war, the danger pressing, dependent princes, dukes, magistrates can declare it by natural law, since it is licit even to any private person to repel force by force. Understand, while danger and necessity last; these ceasing, they ought to seek and await from the supreme prince or independent state the judgment, power and execution of the injury to be avenged.

The just cause of declaring offensive war, generally speaking, is single, namely grave injury or damage inflicted, unrepaired, nor repairable in any other way.

As to right intention, war must not be waged except from intention of justice and common good, as Augustine teaches, Epist. 189⁵ and lib. XXII contra Faustum, cap. 74.

You will ask 2° whether, in doubt about the justice of a war, it is licit to make war.

I answer. 1°. If reparation of injury is at stake, a prince doubtful about the justice of the war cannot declare war; because in doubt, especially of such moment, the safer part must be chosen. If recovery of possession is at stake, the condition of the possessor is better in doubt, and therefore war cannot be moved against him. If neither possess, it is licit to bring war against him who refuses to divide the thing pro rata of the doubt.

[The true reason why, in doubt about the justice of a war, it is not licit to declare it, is because no one acts licitly, when the act can be good or evil, unless he be morally certain of the honesty of his action, as was explained and proved in the treatise on Human Acts, dissert. V, on Conscience.

When neither possesses, and the right is doubtful, it does not seem that this one or that one should be compelled by violence and arms to divide the thing pro rata of the doubt, because, proceeding thus, the one or the other would be despoiled of his right; but the titles invoked on either side are to be examined and weighed, and if the parties cannot compose their litigation between themselves, they ought to commit the decision of the cause to the arbitration of another, whose sentence they promise themselves to stand by. But if the contenders should come forward voluntarily to agree upon dividing the litigated thing between them, so that each abandon part of the right which perhaps belongs to him, this manner of proceeding is certainly to be commended, whereby justice is fully satisfied, but it seems nowise to be imposed.]

I answer. 2°. Those who are subjects of a warring prince, when ordered, can and ought to enroll themselves in the militia, and enrolled to make war, even if they doubt of the justice of the war, forming however for themselves a practical judgment that they may licitly obey and make war; because 1° the superior has a certain right of commanding, nor is he stripped of his right by the subject's doubt. Ergo the subject is bound to obey; for the right of commanding and the obligation of obeying are correlatives. 2° From denial of obedience, while its cause manifest cannot be assigned, authority is enervated in superiors, and the whole government subverted. From these principles therefore the subject can form the practical judgment that when ordered he may licitly make war. Confer what I wrote dissert. V on Human Acts, art. 2, You will ask 3°.

I answer. 3°. Those who are not subjects of the warring prince, in doubt about the justice of the war, cannot serve as soldiers; because by not soldiering they injure no one, since they have not the necessity of obeying like the subjects of whom in the preceding answer; but by soldiering they expose themselves to inflict grave injury upon another prince. The same seems altogether the reason concerning the subjects of the warring prince, if, not ordered, but of their own will, they serve for pay.

However, since the causes of wars are often not manifest, and the justice of a war also often is obscure even to theologians and jurists themselves, from this answer, true elsewhere per se, it would follow that almost all soldiers were blameworthy; for who among them, whether subjects or not, inquires into the causes and justice of the war? Therefore several weighty authors judge that, if their doubt be merely negative, insofar as they have no reasons for or against, and from the probity and prudence of the prince and his counsels they can persuade themselves that they would not move war unless it were just, they may licitly serve.

But those who are ready to make war whether the war be just or unjust, are certainly in the state of damnation, because of the will to make war even unjustly.

You will ask 3° whether there can be a just war on both sides.

I answer. War per se from the nature of the thing cannot be just on both sides, but it can per accidens and from ignorance, or, others say, it cannot be just on both sides materially, well however formally : because it cannot come to pass, according to the truth of the thing, that in one there be right of seeking satisfaction or restitution without there being in the other obligation of satisfying or restituting; for right and obligation are correlatives. Ergo, if he resist, he resists unjustly, and conversely if he resists justly, the other seeks unjustly. But truly, if one ignore the right of the other, he sins not by resisting, and thus per accidens, by reason of ignorance, the war is just on both sides.

You will ask 4° whether in a just war it is licit to kill innocents.

I answer. In a just war it is never licit per se and by intention to kill innocents, unless special authority has been made by God. Yet it is licit per accidens and beside intention. Proof of the first part. 1° From that general command of God, Exod., XXIII, 7: Thou shalt not kill the innocent and just. 2° An innocent cannot be killed in a just war except on account of the guilt of the state. But that is not licit. Ergo. — Proof of the minor. The state cannot be punished by the enemy except in goods over which it has dominion. But the state has not dominion over the life of innocents, but God alone, who granted not the right of the sword to the state except against those who do evil. Ergo.

Under the name of innocents come infants, children unfit for war, women, peasants, plebeians, merchants having nothing to do with war, clerics and religious, cap. de treuga et pace, lib. I Decretal., tit. 34, c. 2. All these, I say, by presumption of law are judged innocent, until the contrary be proved of them.

Proof of the second part: 1° If neither per accidens and beside intention killing innocents were licit in a just war, it would be impossible to carry out a war : a city could not, e.g., be stormed, or a tower demolished in which innocents were enclosed; enemies could not be attacked who placed innocents in the front of battle, which they would never omit, and thus the justice of the combatants would be frustrated. 2° When two effects, equally immediate, of one act, the one good, the other evil, can, a just reason subsisting, the good effect be intended and the evil permitted.⁶ But the demolition of a tower or the explosion of war machines upon enemies is immediately protective of the homeland and destructive of innocents. Ergo the first effect can be intended for the common good, and the other permitted. This principle of the Author more widely drawn out and expounded you have dissert. VIII, On Justice, art. 2.

You will ask 5° what are the obligations and sins of combatants.

I answer. 1°. Supreme princes and leaders of armies 1° are bound ex officio to see that military discipline be observed insofar as they can, and to coerce soldiers from plunderings, thefts, burnings, ravishments, etc.; otherwise they sin mortally and are bound to restitution of those damages. 2° They sin mortally if they provide not just pay to the soldiers, and they are bound to restitute damages which soldiers inflict by lack of pay.

I answer. 2°. Subordinate leaders sin mortally and are bound to restitution : 1° If, no necessity compelling, they rashly expose soldiers to sure slaughter. 2° If they retain part of the pay or moneys conceded for soldiers. 3° If they have a smaller number of soldiers and receive pay for a greater, such are those who, while the troops are mustered, present not real soldiers in military dress, commonly called passe-volants, or present the same twice or thrice : into which sin also those muster-masters or commissaries implicate themselves who knowingly admit it; nor are they bound only for theft of pay, but for the damage which can accrue to the state on account of insufficient number of soldiers. Except how, they shall have seen. 4° Those sin and are bound to restitution who, while they lead troops through a province, on account of accepted gifts from certain villages or citizens, burden other villages, or citizens, with the number of soldiers.

I answer. 3°. Common soldiers 1° ought to be content with their pay, according to the precept of the holy Precursor, Luc., III, 14. Whence they sin mortally and are bound to restitution if, in lands not hostile, beyond roof, bed, straw, etc., as custom allows, they extort moneys, despoil, pillage, or otherwise do damage. Nor does the authority or connivance of leaders excuse them, unless perchance, the pay failing or diminished, they take precisely what is necessary for urgent need. 2° They are bound, even with certain peril of life, not to desert their post, or fortress, unless victory be plainly despaired of.

[I answer. 4°. While war lasts, all those things are licit which are permitted by the law of nations and are necessary for obtaining the end. Hence it is licit to prepare ambushes, employ stratagems, e.g., feign flight, etc. In a just war it is licit to besiege cities and storm them, deprive them of water. Likewise to despoil innocents of goods, if they be members of the enemy state, nor can victory be had otherwise. It is licit even to demolish churches, if enemies use them for defense. Sometimes, but only from the most urgent cause, cities can be delivered to plunder. This however is adverse to modern customs, besides the case where a justly imposed tribute is denied.

But illicit are frauds which can be guarded against by no prudence, as to infect wells and fountains with poison. Nor is it licit to hire assassins who secretly slay the leader of the opposing army; for, by consent of all, perfidy can never be made honorable.

Likewise it is not licit to kill captives taken in war and all who can harm no longer, except in punishment of crime. Captives moreover can flee, if no promise given by them stand in the way.⁷

You will ask 6° whether it is licit for a Catholic prince to call heretics or Gentiles to aid.

I answer. affirmatively per se, at least speaking speculatively, provided scandal and peril of faith be far off. But it can be per accidens illicit, namely on account of damages imminent to religion.

You will ask 7° when do the fighting become irregular.

I answer. In a just defensive war, even those who kill do not become irregular. But in a just offensive war, those alone who kill incur irregularity, not indeed others. But in an unjust war, though only one be killed, all aggressors are irregular, because they act after the manner of one agent.⁸ This last is drawn out from St. Thomas.⁹ Of these things more fully below, on Irregularity.