Summa Th. II-II EN Qu.39


We must now consider the vices contrary to peace, which belong to deeds: such are schism, strife, sedition, and war. In the first place, then, about schism, there are four points of inquiry:

(1) Whether schism is a special sin?

(2) Whether it is graver than unbelief?

(3) Of the power exercised by schismatics;

(4) Of the punishment inflicted on them.

Whether schism is a special sin?

Objection: 1. It would seem that schism is not a special sin. For "schism," as Pope Pelagius I says (Epist. ad Victor. et Pancrat.), "denotes a division." But every sin causes a division, according to Is 59: "Your sins have divided between you and your God." Therefore schism is not a special sin.
2. Further, a man is apparently a schismatic if he disobeys the Church. But every sin makes a man disobey the commandments of the Church, because sin, according to Ambrose (De Parad. viii) "is disobedience against the heavenly commandments." Therefore every sin is a schism.
3. Further, heresy also divides a man from the unity of faith. If, therefore, the word schism denotes a division, it would seem not to differ, as a special sin, from the sin of unbelief.

On the contrary Augustine (Contra Faust. xx, 3; Contra Crescon. ii, 4) distinguishes between schism and heresy, for he says that a "schismatic is one who holds the same faith, and practises the same worship, as others, and takes pleasure in the mere disunion of the community, whereas a heretic is one who holds another faith from that of the Catholic Church." Therefore schism is not a generic sin.
I answer that As Isidore says (Etym. viii, 3), schism takes its name "from being a scission of minds," and scission is opposed to unity. Wherefore the sin of schism is one that is directly and essentially opposed to unity. For in the moral, as in the physical order, the species is not constituted by that which is accidental. Now, in the moral order, the essential is that which is intended, and that which results beside the intention, is, as it were, accidental. Hence the sin of schism is, properly speaking, a special sin, for the reason that the schismatic intends to sever himself from that unity which is the effect of charity: because charity unites not only one person to another with the bond of spiritual love, but also the whole Church in unity of spirit.Accordingly schismatics properly so called are those who, wilfully and intentionally separate themselves from the unity of the Church; for this is the chief unity, and the particular unity of several individuals among themselves is subordinate to the unity of the Church, even as the mutual adaptation of each member of a natural body is subordinate to the unity of the whole body. Now the unity of the Church consists in two things; namely, in the mutual connection or communion of the members of the Church, and again in the subordination of all the members of the Church to the one head, according to Col 2,18-19: "Puffed up by the sense of his flesh, and not holding the Head, from which the whole body, by joints and bands, being supplied with nourishment and compacted, groweth unto the increase of God." Now this Head is Christ Himself, Whose viceregent in the Church is the Sovereign Pontiff. Wherefore schismatics are those who refuse to submit to the Sovereign Pontiff, and to hold communion with those members of the Church who acknowledge his supremacy.

Reply to Objection: 1. The division between man and God that results from sin is not intended by the sinner: it happens beside his intention as a result of his turning inordinately to a mutable good, and so it is not schism properly so called.
2. The essence of schism consists in rebelliously disobeying the commandments: and I say "rebelliously," since a schismatic both obstinately scorns the commandments of the Church, and refuses to submit to her judgment. But every sinner does not do this, wherefore not every sin is a schism.
3. Heresy and schism are distinguished in respect of those things to which each is opposed essentially and directly. For heresy is essentially opposed to faith, while schism is essentially opposed to the unity of ecclesiastical charity. Wherefore just as faith and charity are different virtues, although whoever lacks faith lacks charity, so too schism and heresy are different vices, although whoever is a heretic is also a schismatic, but not conversely. This is what Jerome says in his commentary on the Epistle to the Galatians [*In Ep. ad Tit. iii, 10]: "I consider the difference between schism and heresy to be that heresy holds false doctrine while schism severs a man from the Church." Nevertheless, just as the loss of charity is the road to the loss of faith, according to 1Tm 1,6: "From which things," i.e. charity and the like, "some going astray, are turned aside into vain babbling," so too, schism is the road to heresy. Wherefore Jerome adds (In Ep ad Tit. iii, 10) that "at the outset it is possible, in a certain respect, to find a difference between schism and heresy: yet there is no schism that does not devise some heresy for itself, that it may appear to have had a reason for separating from the Church."

Whether schism is a graver sin than unbelief?

Objection: 1. It would seem that schism is a graver sin than unbelief. For the graver sin meets with a graver punishment, according to Dt 25,2: "According to the measure of the sin shall the measure also of the stripes be." Now we find the sin of schism punished more severely than even the sin of unbelief or idolatry: for we read (Ex 32,28) that some were slain by the swords of their fellow men on account of idolatry: whereas of the sin of schism we read (Nb 16,30): "If the Lord do a new thing, and the earth opening her mouth swallow them down, and all things that belong to them, and they go down alive into hell, you shall know that they have blasphemed the Lord God." Moreover the ten tribes who were guilty of schism in revolting from the rule of David were most severely punished (2R 17,20). Therefore the sin of schism is graver than the sin of unbelief.
2. Further, "The good of the multitude is greater and more godlike than the good of the individual," as the Philosopher states (Ethic. i, 2). Now schism is opposed to the good of the multitude, namely, ecclesiastical unity, whereas unbelief is contrary to the particular good of one man, namely the faith of an individual. Therefore it seems that schism is a graver sin than unbelief.
3. Further, a greater good is opposed to a greater evil, according to the Philosopher (Ethic. viii, 10). Now schism is opposed to charity, which is a greater virtue than faith to which unbelief is opposed, as shown above (Question [10], Article [2]; Question [23], Article [6]). Therefore schism is a graver sin than unbelief.

On the contrary That which results from an addition to something else surpasses that thing either in good or in evil. Now heresy results from something being added to schism, for it adds corrupt doctrine, as Jerome declares in the passage quoted above (Article [1], ad 3). Therefore schism is a less grievous sin than unbelief.
I answer that The gravity of a sin can be considered in two ways: first, according to the species of that sin, secondly, according to its circumstances. And since particular circumstances are infinite in number, so too they can be varied in an infinite number of ways: wherefore if one were to ask in general which of two sins is the graver, the question must be understood to refer to the gravity derived from the sin's genus. Now the genus or species of a sin is taken from its object, as shown above (I-II 72,1; I-II 73,3). Wherefore the sin which is opposed to the greater good is, in respect of its genus, more grievous, for instance a sin committed against God is graver than a sin committed against one's neighbor.Now it is evident that unbelief is a sin committed against God Himself, according as He is Himself the First Truth, on which faith is founded; whereas schism is opposed to ecclesiastical unity, which is a participated good, and a lesser good than God Himself. Wherefore it is manifest that the sin of unbelief is generically more grievous than the sin of schism, although it may happen that a particular schismatic sins more grievously than a particular unbeliever, either because his contempt is greater, or because his sin is a source of greater danger, or for some similar reason.

Reply to Objection: 1. It had already been declared to that people by the law which they had received that there was one God, and that no other God was to be worshipped by them; and the same had been confirmed among them by many kinds of signs. Consequently there was no need for those who sinned against this faith by falling into idolatry, to be punished in an unwonted manner: it was enough that they should be punished in the usual way. On the other hand, it was not so well known among them that Moses was always to be their ruler, and so it behooved those who rebelled against his authority to be punished in a miraculous and unwonted manner.We may also reply by saying that the sin of schism was sometimes more severely punished in that people, because they were inclined to seditions and schisms. For it is written (1 Esdra 4:15): "This city since days gone by has rebelled against its kings: and seditions and wars were raised therein [*Vulg.: 'This city is a rebellious city, and hurtful to the kings and provinces, and . . . wars were raised therein of old']." Now sometimes a more severe punishment is inflicted for an habitual sin (as stated above, I-II 105,2, ad 9), because punishments are medicines intended to keep man away from sin: so that where there is greater proneness to sin, a more severe punishment ought to be inflicted. As regards the ten tribes, they were punished not only for the sin of schism, but also for that of idolatry as stated in the passage quoted.
2. Just as the good of the multitude is greater than the good of a unit in that multitude, so is it less than the extrinsic good to which that multitude is directed, even as the good of a rank in the army is less than the good of the commander-in-chief. In like manner the good of ecclesiastical unity, to which schism is opposed, is less than the good of Divine truth, to which unbelief is opposed.
3. Charity has two objects; one is its principal object and is the Divine goodness, the other is its secondary object and is our neighbor's good. Now schism and other sins against our neighbor, are opposed to charity in respect of its secondary good, which is less than the object of faith, for this is God Himself; and so these sins are less grievous than unbelief. On the other hand, hatred of God, which is opposed to charity in respect of its principal object, is not less grievous than unbelief. Nevertheless of all sins committed by man against his neighbor, the sin of schism would seem to be the greatest, because it is opposed to the spiritual good of the multitude.

Whether schismatics have any power?

Objection: 1. It would seem that schismatics have some power. For Augustine says (Contra Donat. i, 1): "Just as those who come back to the Church after being baptized, are not baptized again, so those who return after being ordained, are not ordained again." Now Order is a kind of power. Therefore schismatics have some power since they retain their Orders.
2. Further, Augustine says (De Unico Bapt. [*De Bap. contra Donat. vi, 5]): "One who is separated can confer a sacrament even as he can have it." But the power of conferring a sacrament is a very great power. Therefore schismatics who are separated from the Church, have a spiritual power.
3. Further, Pope Urban II [*Council of Piacenza, cap. x; cf. Can. Ordinationes, ix, qu. 1] says: "We command that persons consecrated by bishops who were themselves consecrated according to the Catholic rite, but have separated themselves by schism from the Roman Church, should be received mercifully and that their Orders should be acknowledged, when they return to the unity of the Church, provided they be of commendable life and knowledge." But this would not be so, unless spiritual power were retained by schismatics. Therefore schismatics have spiritual power.

On the contrary Cyprian says in a letter (Ep. lii, quoted vii, qu. 1, can. Novatianus): "He who observes neither unity of spirit nor the concord of peace, and severs himself from the bonds of the Church, and from the fellowship of her priests, cannot have episcopal power or honor."
I answer that Spiritual power is twofold, the one sacramental, the other a power of jurisdiction. The sacramental power is one that is conferred by some kind of consecration. Now all the consecrations of the Church are immovable so long as the consecrated thing remains: as appears even in inanimate things, since an altar, once consecrated, is not consecrated again unless it has been broken up. Consequently such a power as this remains, as to its essence, in the man who has received it by consecration, as long as he lives, even if he fall into schism or heresy: and this is proved from the fact that if he come back to the Church, he is not consecrated anew. Since, however, the lower power ought not to exercise its act, except in so far as it is moved by the higher power, as may be seen also in the physical order, it follows that such persons lose the use of their power, so that it is not lawful for them to use it. Yet if they use it, this power has its effect in sacramental acts, because therein man acts only as God's instrument, so that sacramental effects are not precluded on account of any fault whatever in the person who confers the sacrament.On the other hand, the power of jurisdiction is that which is conferred by a mere human appointment. Such a power as this does not adhere to the recipient immovably: so that it does not remain in heretics and schismatics; and consequently they neither absolve nor excommunicate, nor grant indulgence, nor do anything of the kind, and if they do, it is invalid.Accordingly when it is said that such like persons have no spiritual power, it is to be understood as referring either to the second power, or if it be referred to the first power, not as referring to the essence of the power, but to its lawful use.

Reply to Objection: 1. This suffices for the Replies to the Objections.

Whether it is right that schismatics should be punished with excommunication?

Objection: 1. It would seem that schismatics are not rightly punished with excommunication. For excommunication deprives a man chiefly of a share in the sacraments. But Augustine says (Contra Donat. vi, 5) that "Baptism can be received from a schismatic." Therefore it seems that excommunication is not a fitting punishment for schismatics.
2. Further, it is the duty of Christ's faithful to lead back those who have gone astray, wherefore it is written against certain persons (Ez 34,4): "That which was driven away you have not brought again, neither have you sought that which was lost." Now schismatics are more easily brought back by such as may hold communion with them. Therefore it seems that they ought not to be excommunicated.
3. Further, a double punishment is not inflicted for one and the same sin, according to Nahum 1:9: "God will not judge the same twice" [*Septuagint version]. Now some receive a temporal punishment for the sin of schism, according to Question [23], Article [5], where it is stated: "Both divine and earthly laws have laid down that those who are severed from the unity of the Church, and disturb her peace, must be punished by the secular power." Therefore they ought not to be punished with excommunication.

On the contrary It is written (Nb 16,26): "Depart from the tents of these wicked men," those, to wit, who had caused the schism, "and touch nothing of theirs, lest you be involved in their sins."
I answer that According to Sg 11,11, "By what things a man sinneth, by the same also he should be punished" [Vulg.: 'he is tormented']. Now a schismatic, as shown above (Article [1]), commits a twofold sin: first by separating himself from communion with the members of the Church, and in this respect the fitting punishment for schismatics is that they be excommunicated. Secondly, they refuse submission to the head of the Church, wherefore, since they are unwilling to be controlled by the Church's spiritual power, it is just that they should be compelled by the secular power.

Reply to Objection: 1. It is not lawful to receive Baptism from a schismatic, save in a case of necessity, since it is better for a man to quit this life, marked with the sign of Christ, no matter from whom he may receive it, whether from a Jew or a pagan, than deprived of that mark, which is bestowed in Baptism.
2. Excommunication does not forbid the intercourse whereby a person by salutary admonitions leads back to the unity of the Church those who are separated from her. Indeed this very separation brings them back somewhat, because through confusion at their separation, they are sometimes led to do penance
3. The punishments of the present life are medicinal, and therefore when one punishment does not suffice to compel a man, another is added: just as physicians employ several body medicines when one has no effect. In like manner the Church, when excommunication does not sufficiently restrain certain men, employs the compulsion of the secular arm. If, however, one punishment suffices, another should not be employed.


We must now consider war, under which head there are four points of inquiry:

(1) Whether some kind of war is lawful?

(2) Whether it is lawful for clerics to fight?

(3) Whether it is lawful for belligerents to lay ambushes?

(4) Whether it is lawful to fight on holy days?

Whether it is always sinful to wage war?

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 Mt 26,52: "All that take the sword shall perish with the sword." Therefore all wars are unlawful.
2. Further, whatever is contrary to a Divine precept is a sin. But war is contrary to a Divine precept, for it is written (Mt 5,39): "But I say to you not to resist evil"; and (Rm 12,19): "Not revenging yourselves, my dearly beloved, but give place unto wrath." Therefore war is always sinful.
3. Further, nothing, except sin, is contrary to an act of virtue. But war is contrary to peace. Therefore war is always a sin.
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' [*Lc 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 (Rm 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 (Ps 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 (Questions. in Hept., qu. x, super ): "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.
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."
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" (Mt 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."
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. In 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].

Whether it is lawful for clerics and bishops to fight?

Objection: 1. It would seem lawful for clerics and bishops to fight. For, as stated above (Article [1]), wars are lawful and just in so far as they protect the poor and the entire common weal from suffering at the hands of the foe. Now this seems to be above all the duty of prelates, for Gregory says (Hom. in Ev. xiv): "The wolf comes upon the sheep, when any unjust and rapacious man oppresses those who are faithful and humble. But he who was thought to be the shepherd, and was not, leaveth the sheep, end flieth, for he fears lest the wolf hurt him, and dares not stand up against his injustice." Therefore it is lawful for prelates and clerics to fight.
2. Further, Pope Leo IV writes (xxiii, qu. 8, can. Igitur): "As untoward tidings had frequently come from the Saracen side, some said that the Saracens would come to the port of Rome secretly and covertly; for which reason we commanded our people to gather together, and ordered them to go down to the seashore." Therefore it is lawful for bishops to fight.
3. Further, apparently, it comes to the same whether a man does a thing himself, or consents to its being done by another, according to Rm 1,32: "They who do such things, are worthy of death, and not only they that do them, but they also that consent to them that do them." Now those, above all, seem to consent to a thing, who induce others to do it. But it is lawful for bishops and clerics to induce others to fight: for it is written (xxiii, qu. 8, can. Hortatu) that Charles went to war with the Lombards at the instance and entreaty of Adrian, bishop of Rome. Therefore they also are allowed to fight.
4. Further, whatever is right and meritorious in itself, is lawful for prelates and clerics. Now it is sometimes right and meritorious to make war, for it is written (xxiii, qu. 8, can. Omni timore) that if "a man die for the true faith, or to save his country, or in defense of Christians, God will give him a heavenly reward." Therefore it is lawful for bishops and clerics to fight.

On the contrary It was said to Peter as representing bishops and clerics (Mt 16,52): "Put up again thy sword into the scabbard [Vulg.: 'its place'] [*"Scabbard" is the reading in Jn 18,11]." Therefore it is not lawful for them to fight.
I answer that Several things are requisite for the good of a human society: and a number of things are done better and quicker by a number of persons than by one, as the Philosopher observes (Polit. i, 1), while certain occupations are so inconsistent with one another, that they cannot be fittingly exercised at the same time; wherefore those who are deputed to important duties are forbidden to occupy themselves with things of small importance. Thus according to human laws, soldiers who are deputed to warlike pursuits are forbidden to engage in commerce [*Cod. xii, 35, De Re Milit.].Now warlike pursuits are altogether incompatible with the duties of a bishop and a cleric, for two reasons. The first reason is a general one, because, to wit, warlike pursuits are full of unrest, so that they hinder the mind very much from the contemplation of Divine things, the praise of God, and prayers for the people, which belong to the duties of a cleric. Wherefore just as commercial enterprises are forbidden to clerics, because they unsettle the mind too much, so too are warlike pursuits, according to 2Tm 2,4: "No man being a soldier to God, entangleth himself with secular business." The second reason is a special one, because, to wit, all the clerical Orders are directed to the ministry of the altar, on which the Passion of Christ is represented sacramentally, according to 1Co 11,26: "As often as you shall eat this bread, and drink the chalice, you shall show the death of the Lord, until He come." Wherefore it is unbecoming for them to slay or shed blood, and it is more fitting that they should be ready to shed their own blood for Christ, so as to imitate in deed what they portray in their ministry. For this reason it has been decreed that those who shed blood, even without sin, become irregular. Now no man who has a certain duty to perform, can lawfully do that which renders him unfit for that duty. Wherefore it is altogether unlawful for clerics to fight, because war is directed to the shedding of blood.

Reply to Objection: 1. Prelates ought to withstand not only the wolf who brings spiritual death upon the flock, but also the pillager and the oppressor who work bodily harm; not, however, by having recourse themselves to material arms, but by means of spiritual weapons, according to the saying of the Apostle (2Co 10,4): "The weapons of our warfare are not carnal, but mighty through God." Such are salutary warnings, devout prayers, and, for those who are obstinate, the sentence of excommunication.
2. Prelates and clerics may, by the authority of their superiors, take part in wars, not indeed by taking up arms themselves, but by affording spiritual help to those who fight justly, by exhorting and absolving them, and by other like spiritual helps. Thus in the Old Testament (Jos 6,4) the priests were commanded to sound the sacred trumpets in the battle. It was for this purpose that bishops or clerics were first allowed to go to the front: and it is an abuse of this permission, if any of them take up arms themselves.
3. As stated above (Question [23], Article [4], ad 2) every power, art or virtue that regards the end, has to dispose that which is directed to the end. Now, among the faithful, carnal wars should be considered as having for their end the Divine spiritual good to which clerics are deputed. Wherefore it is the duty of clerics to dispose and counsel other men to engage in just wars. For they are forbidden to take up arms, not as though it were a sin, but because such an occupation is unbecoming their personality.
4. Although it is meritorious to wage a just war, nevertheless it is rendered unlawful for clerics, by reason of their being deputed to works more meritorious still. Thus the marriage act may be meritorious; and yet it becomes reprehensible in those who have vowed virginity, because they are bound to a yet greater good.

Whether it is lawful to lay ambushes in war?

Objection: 1. It would seem that it is unlawful to lay ambushes in war. For it is written (Dt 16,20): "Thou shalt follow justly after that which is just." But ambushes, since they are a kind of deception, seem to pertain to injustice. Therefore it is unlawful to lay ambushes even in a just war.
2. Further, ambushes and deception seem to be opposed to faithfulness even as lies are. But since we are bound to keep faith with all men, it is wrong to lie to anyone, as Augustine states (Contra Mend. xv). Therefore, as one is bound to keep faith with one's enemy, as Augustine states (Ep ad Bonif. clxxxix), it seems that it is unlawful to lay ambushes for one's enemies.
3. Further, it is written (Mt 7,12): "Whatsoever you would that men should do to you, do you also to them": and we ought to observe this in all our dealings with our neighbor. Now our enemy is our neighbor. Therefore, since no man wishes ambushes or deceptions to be prepared for himself, it seems that no one ought to carry on war by laying ambushes.

On the contrary Augustine says (Questions. in Hept. qu. x super Jos): "Provided the war be just, it is no concern of justice whether it be carried on openly or by ambushes": and he proves this by the authority of the Lord, Who commanded Joshua to lay ambushes for the city of Hai (Jos 8,2).
I answer that The object of laying ambushes is in order to deceive the enemy. Now a man may be deceived by another's word or deed in two ways.First, through being told something false, or through the breaking of a promise, and this is always unlawful. No one ought to deceive the enemy in this way, for there are certain "rights of war and covenants, which ought to be observed even among enemies," as Ambrose states (De Officiis i).Secondly, a man may be deceived by what we say or do, because we do not declare our purpose or meaning to him. Now we are not always bound to do this, since even in the Sacred Doctrine many things have to be concealed, especially from unbelievers, lest they deride it, according to Mt 7,6: "Give not that which is holy, to dogs." Wherefore much more ought the plan of campaign to be hidden from the enemy. For this reason among other things that a soldier has to learn is the art of concealing his purpose lest it come to the enemy's knowledge, as stated in the Book on Strategy [*Stratagematum i, 1] by Frontinus. Such like concealment is what is meant by an ambush which may be lawfully employed in a just war.Nor can these ambushes be properly called deceptions, nor are they contrary to justice or to a well-ordered will. For a man would have an inordinate will if he were unwilling that others should hide anything from him

Reply to Objection: 1. This suffices for the Replies to the Objections.

Summa Th. II-II EN Qu.39