(b) Interference, however, with the religious rights of others or sinful practices that are injurious to others are a sufficient reason for war. No war ever had a more legitimate cause than the Crusades, which were undertaken to defend the Christian religion against the unspeakable atrocities of infidels. The cause of humanity justifies a war to put an end to such evils as cannibalism or human sacrifice.
1392. Is it lawful to make war on another nation in order to bring to it the benefits of modern civilization? (a) If the uncivilized nation lacks a government and suffers from disorder, it is an act of charity for a civilized nation to set up a government there which will act for the benefit of the people of the country. It is also lawful to make war on those who resist the government thus established. (b) If the uncivilized nation has its own orderly form of government and is at peace, no other nation has the right to interfere under pretext of introducing a higher type of government. Colonial expansion is not a sufficient reason for war in such circumstances.
1393. The following causes for war are not sufficient:
(a) Motives clearly sinful are such as do not suppose any injury done by the other nation, but rather some evil passion of pride, greed, jealousy, suspicion, or selfishness on one’s own side. Hence, it is not lawful to go to war for the glory of a ruler or of the nation, for the enlargement of one’s territory, for the advantage that may be gained over a commercial rival, for the preservation of the balance of power, or for the prevention of difficulties at home.
(b) Motives apparently just, but really sinful, are injuries done by another, if one has secretly provoked them in order to have a pretext for war. It is not right to make war on a people because of attacks made by their citizens, if these attacks were purposely caused by one’s own citizens.
(c) Motives of displeasure with another nation are not sufficient as motives for war, if the other nation has violated no right of justice, but only acted in a way not consonant with charity or friendship. Thus, the fact that one nation denies another financial assistance or the tariff advantages granted to a third nation is not a _casus belli_; for in matters of benevolence or privilege there is no strict claim or title, and hence no right to have recourse to arms.
1394. Is war lawful when the justice of the cause is doubtful? (a) The government may not declare war, unless it is morally certain that right is on its side. The consequences of war are so dreadful, and the use of force against another nation is such an extreme measure, that one should refrain from hostilities as long as one’s moral right is uncertain.
(b) Volunteers not already enlisted may not offer their services to a belligerent, unless they are morally certain that his cause is just. They participate in war from choice, and they should assure themselves that their choice is correct.
(c) Subjects called to the colors should fight for their country, even if they are in doubt about the justice of the cause, for the presumption is on the side of the government. This does not mean, however, that one should be willing to fight for one’s country, right or wrong. nor that one would be obliged to fight for a cause manifestly unjust, or to obey an order flagrantly wrong.
1395. What is the meaning of “moral certitude” in the previous paragraph? (a) Some moralists believe that a high degree of probability of the righteousness of his cause suffices in order that a ruler may take steps towards war. (b) The greater number of moralists, however, hold that no degree of probability suffices. The justifying reasons must be clearer than day, and the state which goes to war must not entertain a single doubt that its cause is right. This opinion we prefer; for, if a jury may not sentence an accused to death as long as there is a reasonable doubt of his innocence, neither ought a nation to pass what is really a death sentence on hundreds or thousands of citizens as long as there exists a doubt of a compelling reason for such a course. It should, however, be observed that a ruler who has only probable evidence that an injury has been done already, may have certainty that it will be done, if it is not prevented by war.