1386. In order that the cause of war be just, it is necessary that the enemy nation has done or now menaces an injury which cannot be repaired without war, and which is so serious that the evils of war are less than that of toleration.
(a) Thus, a serious injury or grave dishonor inflicted by another nation is the only just cause for the armed conflict which constitutes war, for war is exercised as a punishment or a compulsion, and these are unjust if no grave and formal fault is supposed.
(b) Only an injury that cannot be otherwise repaired is a just cause for war, because a state has no right to use force against another sovereign state except as a last resort. Hence, if the country at fault has already made satisfaction or has promised to make satisfaction, war should not be declared.
(c) Only an injury so grave that it outweighs the risks and losses of war is a justification for making war, for when two effects, one good and one evil, follow from an act, there must be a proportionately grave reason for permitting the evil effect before acting (see 104, 105). It would be wrong to avenge some small insult or some isolated injury at the expense of immense treasure and enormous loss of life. Modern warfare is so devastating that only the gravest reasons known to society can authorize it. For, according to scientists, a single H-bomb may cause death and destruction over a wide area, perhaps the space of a hundred square miles. In view of the havoc which is foreseen to outweigh the benefits of victory, it could happen that a nation with justice on its side and the potential to wage war would nevertheless not be justified in waging war (see 1410). This destructive power of modern weapons, however, need not imply a sweeping condemnation of all warfare. Spiritual values, e.g., freedom from tyranny, freedom to worship God, still hold primacy over material values and can be deemed so precious as to outweigh the great loss of lives and property involved in defending them or recovering them through modern warfare. “A people menaced by, or already victims of unjust aggression, if it desires to think and to act in a Christian manner, cannot remain in passive indifference” (Pope Pius XII, Christmas Message of 1948).
1387. In comparing the advantages and disadvantages of war, one should take into consideration, not only the losses oneself will suffer, but also the losses that will be suffered by others. (a) Thus, if the enemy nation will be ruined as the price of one’s obtaining some small right, charity would urge that one abstain from war. (b) If the world in general or posterity will suffer greater evils materially or spiritually than a nation is now suffering from the denial of some non-essential right, charity at least should rule out a declaration of war.
1388. Is there a just reason for war, when a fault has been committed on both sides? (a) If the injuries are about equal and still in being, there is no reason for war, for neither nation is in a position to accuse the other of injustice. (b) If the injuries are quite unequal or one nation has shown a willingness to cease from injury, the less guilty nation has a right to make war; but it should first clear itself of injustice, before it proceeds to chastise injustice in the other.
1389. Sufficient causes for making war are: (a) grave injury to the honor of a nation, such as insult to its ruler or ambassadors (II Kings, x.); (b) injury to the natural right of the nation to existence, self-preservation, property, free action within its own sphere; thus, a people may make war to defend their independence (I Mach., iii. 59), to recover territory taken from them unjustly, to resist a violation of neutrality (II Kings, viii. 5), to protect their own citizens and commerce; (c) injury to the rights of the nation under positive law. Thus, a nation may make war to uphold important international agreements, to enforce the observance of treaties, and the like.
1390. Injury done to a third nation or to the subjects of a third nation may also be a sufficient reason for war. (a) Thus, out of justice, a nation is obliged to help its allies in a just war; for to help those with whose interests one’s own interests are involved is only self-defense. (b) Out of charity, a nation that has the right of intervention may lawfully go to war to protect a weaker nation against a stronger and bullying nation, to assist a government unjustly attacked by its subjects, or to help innocent subjects who are tyrannized over by their government.
1391. Is it lawful to go to war over religion or morality?
(a) Error in the religion or immorality in the practices of another people is not a sufficient reason for making war on them. No one can be forced to believe, says St. Augustine; and it is likewise true that no one can be forced to love virtue, whereas external conformity without conviction or love is hypocritical. Moreover, a nation has no authority to correct the sins of those not subject to it. Hence, it would not be right to attack a people for the sole reason that it was pagan or polygamous.