1402. It is not true that all is fair in war, for even a just cause cannot sanction unjust means. The commandments of God and the laws of nations retain their force even amid the clash of arms. Examples of acts of war that are unlawful, as being opposed to the natural law are the following: (a) acts of irreligion, such as wanton destruction of churches or monasteries; (b) attempts to seduce enemy soldiers from the obedience or loyalty owed their commanders; (c) murder, that is, the direct killing of innocent and unarmed persons, as when one refuses quarter to soldiers who wish to surrender, fires on an officer bearing a flag of truce, sinks passenger ships not engaged on errands of war, massacres the civil population by raids from the air, places a defenceless population at the mercy of savages or criminals employed as soldiers; (d) the dishonoring of women, the establishment of brothels for soldiers; (e) stealing, such as the unauthorized pillage of a town or countryside; (f) lying, such as breaking treaties, not keeping faith with the foe, entering into perjured agreements, circulating false stories of atrocities, forging of documents, etc.
1403. Just war is resistance to unjust aggression, and so the same means are lawful in warfare as are lawful in private aggression. (a) Thus, the means used against an aggressor must not be evil in themselves, as when a person protects himself against a murderer by making an innocent person a shield. Hence, in war one may not use any means that is opposed to the law of God, or to human contracts or other obligations. (b) The means employed must be such as are really necessary for overpowering the aggressor. Thus, it is not lawful to kill a burglar when wounding him will suffice for the protection of one’s property. Likewise, in war it is not lawful to exterminate or depopulate an enemy, if the end of war can be attained by depriving the enemy of his weapons.
1404. The principal classes of acts of war from the moral standpoint are: (a) acts in which violence is done to things connected with religion; (b) acts of violence against persons; (c) acts of violence against property; (d) acts used to conceal truth.
1405. Acts of War and Sacred Times.—(a) It is lawful to carry on warfare, offensively or defensively, on feasts, when this is necessary, just as it is lawful to do servile work on those days in case of necessity (I Mach., ii. 41; John, vii. 23). (b) But if a suspension of hostilities can be arranged for feast days (especially for the greater ones, such as Christmas and Easter), warfare should be discontinued at those times.
1406. Acts of War and Sacred Places.—(a) It is lawful to attack a church building, if it is certainly being used for military purposes. It is also lawful to attack fortifications, and thus unintentionally to harm adjacent church buildings. (b) It is not lawful, apart from these reasons of real military necessity, to injure sacred places or edifices.
1407. Acts of War and Sacred Persons.—(a) It is lawful for clerics to cooperate in a just war in spiritual ways, as by exhortations, prayers, and religious ministrations. Moses prayed for the armies of Israel during battle (Exod., xvii. 8 sqq.), the priests accompanied Josue around the wall of Jericho (Jos., vi. 4), and St. Bernard and other holy men preached crusades. (b) It is not lawful, apart from necessity (as in case of conscription), for clerics to take part in actual fighting. Warfare is unbecoming in a cleric, because he is enrolled for a spiritual warfare (II Tim., ii. 4), and because his leader, Christ, shed His own blood, not that of others (Matt, xxvi. 52). Hence, the Church forbids clerics to volunteer as soldiers (Canon 141).
1408. The persons to whom violence is done during war are: (a) Combatants, that is, all those who are engaged in the actual promotion of the war. Direct combatants are the fighters, such as the officers and privates of army, navy, and air force; indirect combatants are the unarmed auxiliaries of the soldiers in military ways, such as makers of munition, transporters of supplies, and those in the communication service. (b) Non-combatants are enemy subjects who are neither fighters nor auxiliaries of the armed forces, such as chaplains and members of the medical service in the army, persons in civil life and occupation, old men, women, and children. (c) Neutrals are those who are not subject to either of the warring contenders, and who take no part in the hostilities, although they may sympathize with one side.
1409. The Killing or Wounding of Enemy Combatants.—(a) According to natural law, it is lawful to kill or wound the enemy in battle, or to starve him by blockade, just as it is lawful in self-defense to kill or wound an unjust aggressor. (b) According to international law, it was expressly forbidden to attack in ways that make war more cruel without hastening the decision.
1410. The Killing or Wounding of Non-Combatants.—(a) The indirect killing of non-combatants (i.e., killing which is unintentional and unavoidable) is lawful, according to the rules given for double effect (see 103, 104). Hence, it is lawful to bombard the fortifications, arsenals, munition works, and barracks of a town, to sink passenger liners that are carrying arms or stores to the enemy, to cut off food supplies from a town or country in order to starve out its troops, although these measures will entail the deaths of some civilians as well as of combatants. Humanity requires, however, that an effort be made to spare the non-combatants, when possible, as by serving warning of attack, so that they may be removed to safety. When it is a question, however, of the use of modern weapons (the atom, hydrogen or cobalt bombs) on military targets in the vicinity of large cities, where it is foreseen that many thousands of civilians will be killed or severely wounded, then the principle of double effect seems to rule out the lawfulness of using such devastating weapons. The immediate evil effect, the slaughter of the innocents, could hardly be called incidental and only reluctantly permitted. Concretely, the inevitable results of the use of such weapons would have to be intended directly, if not as an end, at least as a means.
(b) The direct killing of non-combatants (i.e., killing which is intentional) is unlawful and constitutes the sin of murder. Obliteration bombing, the dropping of H-bombs or atom bombs on a residential section of a city containing no military objectives, are of this character; for they are attacks on civilians. It can not be argued that such an attack would probably break down the morale of the citizens to such an extent that they would force their rulers to make peace and so save many thousands of lives. For this argument is based on the principle that a good end justifies evil means.