(WARNING: There are some graphic descriptions of violence in this post.)
It's not unusual for a comment thread to center on a minor point within a post. That happened here, when I made a parenthetical remark about Kevin Miller describing the Hiroshima and Nagasaki bombings as immoral. "I am open to the argument," I wrote, "but since the entire Japanese population was being mobilized for war, I tend to think it was legitimate."
Most of the people who commented were Catholic Light regulars, and I have to say, I'm a little disappointed in you guys. Instead of responding to my invitation to discuss the matter, you were sarcastic, belittling, and one person even implied I am Protestant (not that there's anything wrong with that!), or at least I think like one. Kevin was the only one who responded in a serious, reasonable manner. I deleted a couple of comments that I can only assume were written rashly.
Before I explain my opinion fully (and it is merely an opinion) about our nuclear strikes on those two cities, I will define my boundaries. I am not arguing for a blanket approval of all American military action, and I am quite capable of recognizing when our nation has not used force properly or with moral rectitude. Those occasions include the Mexican War, probably the Spanish-American War (though we were intervening to help freedom fighters), and just about every small intervention in the Western Hemisphere from 1900-1940, e.g. Nicaragua, Haiti, etc.
Still less do I want to defend the Anglo-American strategic bombing campaign in World War II. In practical terms, bombing German cities to rubble did very little to end the war, as it intensified German resistance. It didn't disrupt their war production as much as one might think, either. The Allies found the German war planning headquarters and obliterated it during one ferocious raid, but production soared after that. Production only slowed when the Allies started capturing German-held territory. In human terms, the campaign was an unmitigated disaster. While the motivation was understandable, given that thousands of innocent British subjects died in indiscriminate V-1 and V-2 bombings, the explosives dropped on the Nazi empire were often motivated by malice, not military necessity. Until the last few months of the war, RAF and U.S. Army Air Corps crews suffered casualty rates similar to the Marines and soldiers in the Pacific theater. This campaign was thus neither militarily nor morally justifiable.
In Catholic teaching, it can be licit, under certain very limited circumstances, to bring about the death of the innocent if their death is incidental to a morally good end. This is the "principle of double effect." This does not always refer to accidental death: it is licit, for example, to administer painkillers to terminally ill patients that will certainly hasten death, if the willed end is to palliate the pain and not to kill.
The Catechism spells this out explicitly in the matter of lethal force:
(2263) The legitimate defense of persons and societies is not an exception to the prohibition against the murder of the innocent that constitutes intentional killing. "The act of self-defense can have a double effect: the preservation of one's own life; and the killing of the aggressor.... The one is intended, the other is not."
(2264) Love toward oneself remains a fundamental principle of morality. Therefore it is legitimate to insist on respect for one's own right to life. Someone who defends his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow:
If a man in self-defense uses more than necessary violence, it will be unlawful: whereas if he repels force with moderation, his defense will be lawful.... Nor is it necessary for salvation that a man omit the act of moderate self-defense to avoid killing the other man, since one is bound to take more care of one's own life than of another's.
(2265) Legitimate defense can be not only a right but a grave duty for someone responsible for another's life. Preserving the common good requires rendering the unjust aggressor unable to inflict harm. To this end, those holding legitimate authority have the right to repel by armed force aggressors against the civil community entrusted to their charge.
The passage above in bold seems to be the crucial one, for it gives us the right way to frame this issue: was the destruction of Hiroshima and Nagasaki "more than necessary violence"? Let us state plainly what the bombings were — the use of massive, overwhelming force against two population centers, with the certainty that many innocent lives would be lost. The object was not merely to destroy military infrastructure, or to kill military personnel, but to show the necessity of immediate surrender. Something like 300,000 people were killed, including thousands who in no way could be considered legitimate targets.
Although these were the first times that atomic bombs were used in war, it was hardly the first time war caused mass death among the innocent. People seem to imagine that modern warfare was uniquely destructive, and in may ways it is; gunpowder and bullets are very efficient technologies for killing. Yet most armies in history, if they were campaigning more than a few score miles away from their garrisons, foraged for food in the countryside, devouring crops and livestock at will. The armies of the Thirty Years' War (1618-1648) knew that by emptying the granaries of German peasants, they were condemning multitudes of innocent people to slow, certain death.
To properly consider this question, it helps to consider some of the relevant facts:
1. The cities of Hiroshima and Nagasaki had legitimate military targets within them, and were central to what remained of the Japanese war effort.
2. As I mentioned, the Japanese warlords were training anyone who was capable of giving resistance to the American invaders. They accepted that they could not win the war, but they were prepared to let everyone in Imperial Japan die rather than surrender to the Americans.
3. The warlords could do this because Japanese society was a totalitarian society, probably more completely totalitarian than any other society before or since. Because of Japan's geography and history, the Imperial government dictated practically every aspect of daily life. Fealty to the Emperor was total; no mortal was allowed to look at him, and he was revered as divine. A maximalist, fascist version of bushido, the ancient samurai's code, placed an extreme emphasis on warfare, and every Japanese citizen was expected to support the military effort. This was not a temporary condition, as in the wartime mobilizations of Britain and the America. Unless this religious-military regime was utterly and completely crushed, it would almost certainly reassert itself — particularly if it suffered a humiliating but partial defeat.
4. This totalitarian mentality translated into a complete disregard for human life, and spawned the death-cult of the kamikaze. It meant that Japanese soldiers would not surrender even if they were surrounded by Marines and had no possible hope of escape. It meant that they would do practically anything to kill Americans, including feigning death and then attacking, or pretending to surrender and blowing themselves up when their enemy got too close. If you get a chance, read some of the first-hand accounts of the Pacific campaign. There's one particularly graphic account in "Flags of Our Fathers" where one of the men who raised the flag on Iwo Jima finds the body of his friend, a fellow Navy corpsman, who was tortured to death by the Japanese. His corpse was almost unrecognizable, and his genitals were severed and stuffed into his lifeless mouth.
5. Speaking of war crimes, in "American Caesar," his biography of General Macarthur, William Manchester describes the recapture of Manila, which included the deliberate destruction of Catholic churches and desecrations of the Eucharist (many Filipinos bravely tried to consume it before they were slaughtered.) Japanese soldiers gouged out babies' eyes in front of their parents. This was entirely in keeping with Japanese ideology, which taught that they were a master race and all other peoples were more or less subhuman. There were many similar incidents in Manchuria, Korea, and much of the Pacific Rim, long before the Americans intervened.
6. Removing the Japanese from Okinawa took something like 180,000 American casualties. Even after four years of the bloodiest and most destructive war in human history, that total shocked the U.S. military and public. War planners began talking about a million casualties to capture the Japanese mainland, even a million dead. Japanese deaths would have been in the millions, if not the tens of millions.
Killing a third of a million people is an unimaginable horror, and one that could not be morally acceptable under almost any circumstance. That being said, given the likelihood that many more people would have died in a conventional attack, and that nothing less than a lethal blow against Imperial Japan would have destroyed the threat to world peace that it represented, I believe that the direct attacks on Hiroshima and Nagasaki were justified as a proportional response to end a Japanese-instigated war and the murderous political theology that caused it.
ADDENDUM: Richard asks in another comment thread whether Hiroshima and Nagasaki were selected for military reasons. Here is an explanation of the target selections, written soon after the war:
Hiroshima was a city of considerable military importance. It contained the 2nd Army Headquarters, which commanded the defense of all of southern Japan. The city was a communications center, a storage point, and an assembly area for troops. To quote a Japanese report, "Probably more than a thousand times since the beginning of the war did the Hiroshima citizens see off with cries of 'Banzai' the troops leaving from the harbor"....
The city of Nagasaki had been one of the largest sea ports in southern Japan and was of great war-time importance because of its many and varied industries, including the production of ordnance, ships, military equipment, and other war materials. The narrow long strip attacked was of particular importance because of its industries....