So, that’s the short answer. But the question deserves a longer answer, and mine would emphasize the following points.
First, whatever one thinks of the atomic bombings, there can be no question of moral equivalence between the Allied and Axis powers. The Japanese empire of the World War II era was an evil empire and the American empire (if one wants to call it that) was not and is not. In particular, while war crimes were committed by individual American soldiers and by some American officials during World War II, they were the exception to the rule. By contrast, war crimes – such as the mass slaughter and rape of civilians, and the torture, enslavement and mass murder of POWs – were the standard practice of the Japanese empire.
Second, and for that reason, Japanese militarism was a grave evil which needed to be destroyed. It is hard to see how that could have been accomplished without the Allied policy of demanding unconditional surrender from the Japanese. Hence while some critics of the atomic bombings are also critical of the policy of unconditional surrender, it seems to me there was good reason for it. Nor does it seem plausible to hold that Japanese resistance was significantly increased by the policy. As anyone familiar with the history of the war in the Pacific knows, the tenacious “to the death” attitude of the Japanese soldier was a deeply ingrained feature of Japanese military culture, and would have been a factor whether or not the Allies had ever demanded unconditional surrender. Indeed, it was a deeply ingrained feature of Japanese culture in general. And so it is no doubt true that destroying Japanese militarism would have required an extremely bloody invasion.
Third, for that reason it is probably true that the atomic bombings saved many lives, both Allied and Japanese, that would have been lost in an invasion. It is also probably true that it saved the lives of POWs like Zamperini. Given Japan’s wicked “kill-all” policy of massacring POWs before they could be liberated – which had been carried out already many times in other parts of Japan’s empire – it is likely that only the abrupt end to the war the shock of the bombings made possible could have prevented the implementation of that policy in the home islands. Critics of the bombings should not pretend otherwise: If they hold (as they should) that we should never do what is intrinsically evil, regardless of the consequences, then they should admit that Hiroshima and Nagasaki force them to put their money where their mouths are, if any real-world example does. (That is not to say that there wasn’t a third option, such as an exhibition bombing or dropping the atomic bomb on an unambiguously military target. But it is at least debatable whether that would have had the same psychological effect.)
Fourth, Zamperini, like most POWs, does seem to think that the bombings saved their lives, and seems also to approve of the bombings. At the same time, Zamperini is not at all glib about this. As Hillenbrand’s book makes clear, Zamperini and other POWs, though they suffered unimaginably under the Japanese, nevertheless found themselves deeply moved upon their liberation by the condition of the Japanese civilian population – by the starvation and disease the population was enduring by war’s end, and by the flattened city after flattened city the POWs saw on their exit from the camps. The POWs were glad to be liberated; they knew that many Japanese supported what their military had been doing (though as Hillenbrand recounts, there were also Japanese soldiers and civilians who did not approve, and who showed kindness to the POWs); and they seem generally to have judged that the bombings were justified. But their attitude was not in general one of vengeance-seeking, either toward the Japanese population in general or even toward their captors. As Hillenbrand recounts, after Allied supplies started to reach the camp Zamperini was in, the POWs shared them with Japanese civilians in the surrounding town. In general, they showed no interest in lynching the guards who had been so cruel to them, and even immediately forgave many of them. They were also evidently troubled by the fact that so many Japanese cities had been razed to the ground. In short, Zamperini and the other men who actually suffered in the POW camps were not as bloodthirsty as the reader who asked about Zamperini’s opinion of Hiroshima assumes they were (or as the reader himself evidently is).
Fifth, while I believe the atomic bombings were immoral, I do not presume to judge the POWs or other American soldiers who think otherwise. I believe they are very gravely mistaken, but I certainly do not believe that they are in general motivated by cruelty, or malice, or an evil ideology comparable to Nazism, communism, Japanese militarism, Ba’athism, or jihadism. As I have indicated, while American policy is sometimes flawed and while isolated war crimes have sometimes been committed in its execution, it is not in general intrinsically evil, as the policies of America's enemies in recent history – Nazi Germany, Imperial Japan, Soviet Russia and other communist states, Saddam’s Iraq, and Al Qaeda and other Islamic terrorist organizations – have been. Those who badmouth the United States and her military while ignoring, minimizing, excusing, or “understanding” the crimes of her enemies deserve only contempt.
Sixth, the case at hand illustrates as vividly as possible that it can be hard to do the right thing, so hard that even otherwise decent men can get themselves to believe that it should not be done. To be sure, it is not quite as hard as people often pretend it is, once we get clear on exactly what the opposite course of action entails. Suppose that for some reason the war could have been ended earlier, and the lives of hundreds of thousands saved, only by torturing and killing a certain four-year-old child in front of his parents. I think even Henry Stimson and Harry Truman, not to mention the tough guys who haunt blog comboxes, would have balked. It is easier to murder the innocent when we think of them in terms of collectivist abstractions – say, by talking vaguely about what “the Japanese” did and what “they” therefore deserve, as opposed to speaking more accurately of what certain specific Japanese military and governmental personnel did and what they deserve, which has very different implications. It is also easier to approve of murdering the innocent when one doesn’t have to do it oneself. If those who approve of Hiroshima and Nagasaki were to describe the acts in question honestly and with precision – as acts of intentionally killing innocent civilians for the sake of terrorizing the survivors into surrendering – and then to imagine themselves behind the bombsight, maybe they would find it much easier to do the right thing and refrain from bombing, whatever the consequences.
Still, even when one is crystal clear about what the right thing to do is, it can be difficult to do it when the costs are high enough. This, as J. Budziszewski argued several years ago in an important article on natural law theory, is why revealed theology is indispensable to practical moral life. The natural law demands our unconditional surrender to its directives, but it does not guarantee that the suffering we sometimes endure for the sake of doing what is right will be compensated. For that reason, it can sometimes seem onerous, even crushing. It is immeasurably easier to do what is right when we know that the Lawgiver Himself will reward us for doing so – but this we can know, or can know clearly anyway, only through divine revelation rather than mere philosophy.