80 The horror and perversity of war is immensely magnified by the addition of scientific weapons. For acts of war involving these weapons can inflict massive and indiscriminate destruction, thus going far beyond the bounds of legitimate defense. Indeed, if the kind of instruments which can now be found in the armories of the great nations were to be employed to their fullest, an almost total and altogether reciprocal slaughter of each side by the other would follow, not to mention the widespread devastation that would take place in the world and the deadly after effects that would be spawned by the use of weapons of this kind. All these considerations compel us to undertake an evaluation of war with an entirely new attitude. The men of our time must realize that they will have to give a somber reckoning of their deeds of war for the course of the future will depend greatly on the decisions they make today. With these truths in mind, this most holy synod makes its own the condemnations of total war already pronounced by recent popes, and issues the following declaration. Any act of war aimed indiscriminately at the destruction of entire cities of extensive areas along with their population is a crime against God and man himself. It merits unequivocal and unhesitating condemnation. The unique hazard of modern warfare consists in this: it provides those who possess modern scientific weapons with a kind of occasion for perpetrating just such abominations; moreover, through a certain inexorable chain of events, it can catapult men into the most atrocious decisions. That such may never truly happen in the future, the bishops of the whole world gathered together, beg all men, especially government officials and military leaders, to give unremitting thought to their gigantic responsibility before God and the entire human race.
Source: Gaudium et Spes (Vatican.va)