A friend of mine showed me a statement made by a priest claiming the use of atomic weapons against Japan was justified. I can understand how some people can believe it to be true... 20 years ago, I believed it (much to my regret).
But that was before I knew the what and why of the Church teaching against it.
But when the Church teaching says...
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.(1) 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,(2) 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. (Gaudium et spes #80 emphasis added)
...then it follows no defense of these atomic attacks can ever be justified by Catholic teaching.
Not Double Effect
Sure, some may try to justify it by double effect, pointing out that countless more may have died without it, but that is misapplied. Double effect requires that the death of innocents be unintended -- something definitely not the case with an indiscriminate weapon that destroyed an entire city.
Hypothesis Contrary to Fact Fallacy
Also, it argues using the fallacy of hypothesis contrary to fact. There were indeed claims (based on the Japanese government wanting to mobilize the entire population for a suicidal defense) that the invasion of the home islands would cause over a million casualties -- but that's assuming that all the civilians would agree.
As my friend pointed out...
"This idea really creates an unrealistic stereotype of the Japanese which serves the purposes of the person making the argument. The arguer turns the Japanese people into a group of mostly hardcore fanatics who are ready and willing to throw themselves vainly at the Allied troops without much thought for personal safety. In essence, they are dehumanized - turned into caricatures of people, without any thought of things like life, family, or future. The Japanese people become almost incapable of individual thought in this scenario. It is both insulting and unrealistic. There is no doubt that some Japanese would have resisted in this fashion, but by no means would it be the situation conjured up by proponents of the bombings."
Shifting the Burden of Proof
Also, I've seen arguments demanding, "tell us what should have been done differently!" But that's shifting the burden of proof. If one alleges the use of these weapons was moral, that person has the burden of proof to show it to be moral.
Because the Catholic Church teaches that these weapons are "a crime against God and man himself," to defend the use of atomic weapons is to deny the authority of the Church to teach on the subject. Perhaps even denying that anything not using extraordinary infallibility is binding.
But before they do, they should consider the words of Pope Pius XII in Humani generis #20...
Nor must it be thought that what is expounded in Encyclical Letters does not of itself demand consent, since in writing such Letters the Popes do not exercise the supreme power of their Teaching Authority. For these matters are taught with the ordinary teaching authority, of which it is true to say: "He who heareth you, heareth me"; and generally what is expounded and inculcated in Encyclical Letters already for other reasons appertains to Catholic doctrine. But if the Supreme Pontiffs in their official documents purposely pass judgment on a matter up to that time under dispute, it is obvious that that matter, according to the mind and will of the Pontiffs, cannot be any longer considered a question open to discussion among theologians.
When the Pope or an ecumenical council uses the ordinary magisterium to teach, it is not a matter where we can go against the Church.