Gaudium et Spes 80 addresses the concerns of the 60’s:
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 deviation that would take place in the world and the deadly after effects that would be spawned by the use of weapons of this kind.
The basis for the “intrusion” of the Church into matters of war and peace. Perhaps John XXIII’s Pacem in Terris is worthy of a separate examination. Still.
All these considerations compel us to undertake an evaluation of war with an entirely new attitude.(Cf. John XXIII, encyclical letter Pacem in Terris, April 11, 1963: AAS 55 (1963), p. 291; “Therefore in this age of ours which prides itself on its atomic power, it is irrational to believe that war is still an apt means of vindicating violated rights.”) The (people) 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,(2. Cf. Pius XII, allocution of Sept. 30, 1954: AAS 46 (1954) p. 589; radio message of Dec. 24, 1954: AAS 47 (1955), pp. 15 ff, John XXIII, encyclical letter Pacem in Terris: AAS 55 (1963), pp. 286-291; Paul VI, allocution to the United Nations, Oct. 4, 1965.) and issues the following declaration.
I don’t think a Church teaching can be put any more explicitly than this:
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 (humankind it)self. It merits unequivocal and unhesitating condemnation.
So much for the hydrogen bomb. But there’s more:
The unique hazard of modern warfare consists in this: it provides those who possess modem scientific weapons with a kind of occasion for perpetrating just such abominations; moreover, through a certain inexorable chain of events, it can catapult (people) into the most atrocious decisions. That such may never truly happen in the future, the bishops of the whole world gathered together, beg all (people), especially government officials and military leaders, to give unremitting thought to their gigantic responsibility before God and the entire human race.
It seems undeniable that the combination of modern weaponry and personal sin will eventually come back to bite someone. On vacation, I was reading that the government’s persecution of Robert Oppenheimer was the occasion for serious breaches in security, not by scientists, but by government officials themselves. British scientists later conceded there was just enough material in the public record to close the gaps in their own hydrogen bomb research. Additionally, the US government’s endorsement of international atomic power gave many nations the means to produce necessary materials for atomic weaponry.