I have contended that such a justification is ludicrous and an example of rationalizing special pleading of the worst kind. It simply can't be sustained, given nuclear technology, even if relatively unknown at the time. If we grant this "lessening of culpability due to ignorance" for Hiroshima, we still could not do so for Nagasaki, since it was then known what would happen.
Furthermore, the target areas (the centers of both cities) and the knowledge that the effect of the bombs spread outwardly from ground zero to cover a known area of a particular estimated size, and the obvious fact that cities contain citizens and population (read: people, persons, human beings; read, "including many elderly men, women, and children"), make it quite implausible (I think, virtually impossible) to maintain that those who ordered such a strike were targeting only military installations and not also people.
Moreover, terror bombing or carpet bombing of cities with known effects had already occurred (particularly in Dresden, Germany and in Tokyo). It is equally absurd to argue that those strikes were not intended to target populations as well as military production facilities, bases, etc. This being the case, and the immediate precedent, it is difficult to make an argument that somehow Hiroshima and Nagasaki were instances of a strike wholly different in nature and intent from Dresden and Tokyo.
If indeed only the military targets were in mind, then why drop these horrific weapons on the center of the city? Was that the way to take out the most military targets? I have read that the bulk of these targets were on the outer peripheries of the city, not in the center. If the goal was to minimize civilian casualties, is not the center of the city the very worst selection for this end? The intent was clearly to kill as many people as possible along with taking out whatever military targets were there. Of course, those who do such an evil thing will maximize the military significance and minimize the human cost, but the facts remain what they are and cannot, in my opinion, be gotten over.
To sum up, then:
1) Deliberately killing civilians in wartime is wrong.I think it is impossible and ludicrous to maintain that 200,000 deaths as a result of a bomb of mostly-known proportions and effects were not "willed," but merely "permitted". How can one drop such a weapon and claim to not know that such a result in human cost would happen? As I stated previously, one can't simply play word games and act as if the magic words "military target" wrap up all the difficulties of this cynical, downright malicious (from the standpoint of the victims) reasoning.
2) The bombings of Hiroshima and Nagasaki did that (to the tune of 200,000 + casualties).
3) It cannot plausibly be maintained that this was a non-intended effect of a moral good (taking out military installations) because of the nature of the weapon, where it was targeted, and immediate historical precedent.
4) Therefore, it was an evil act, since contrary to Catholic just war criteria, and inability of justification by this criterion of double effect.
5) Conclusion: the bombings violate double effect by being morally bad and not at all indifferent.
The second sentence is also quite applicable and damaging to the proponents' position, thought not fatal in and of itself, due to the nature of probabilistic or contingent or theoretical calculations of what would happen in the future. As many high military figures felt at the time (Eisenhower, MacArthur, Nimitz, Halsey, Carter Clarke: who was in charge of the radio intelligence summations; also former President Hoover, etc.), surrender could have been achieved by the end of 1945 without recourse to such horrible measures. There was certainly enough opinion and evidence along these lines for us to have at least more seriously considered such an option, in accord with this principle.
Or else, I suppose proponents could simply argue that all these military experts were ignorant, misguided, acting on emotions alone (as if the pro-bombing crowd did not have a large amount of emotions clouding their reasoning, too), or that they only reported their supposed feelings anachronistically, and after the fact (due to the Cold War, the second-guessing of hindsight, etc.). Some people seem to consider themselves able (as a non-military and non-academic persons), 60 years after the fact, to better make such judgments than folks like Eisenhower and MacArthur. Is that not exceptionally presumptuous?
If we grant that President Truman acted in good faith, with good motives (as I freely do), then we must also grant that those opposed to the decision at the time were equally in good faith, with roughly equal the information needed to make the decision one way or the other. This was not a unanimous consensus at the time, however one wishes to slant or spin it. In other words, many thought that we could achieve the good end without the bad effect of what the bomb in fact caused. it was a viable option; therefore it should have been seriously considered and (I say) chosen as the more moral option of the two.
The good end desired (the surrender and thus the end of the wanton slaughters of the Japanese army and suicides even of Japanese civilians) was promptly achieved. So far so good. The good effect was also produced by the action; yet it was also produced by the bad effect of the action (the mass killings convinced the Japanese of the futility of continuing). Thus, this would seem to suggest that Hiroshima and Nagasaki do not meet this particular just war criterion.
And thus we arrive at the crux of the matter: the previous state of affairs being true; therefore a good end was achieved by a bad means, and this is not allowed in Catholic moral theology. Therefore, the bombings are unjustifiable once again, on this separate and distinct ground. If, to speak theoretically, somehow we had had the technology then to wipe out 200,000 Japanese soldiers, and very few civilians as unintended "collateral damage," then the act would have been quite acceptable by these standards. But since the dead were some 95% civilians, the rationale doesn't wash (as that is the very opposite ration of what should be for this to be a moral act). It was, therefore, an evil act. And one must not commit eveil, even for the achievement of a great good. Such is not Christian ethical thought. Period.
As in the previous example, it cannot be reasonably argued that the bombings were morally good or neutral acts themselves. The double effect principle itself rules out using a bad, evil, immoral act for the purpose of producing good effects, no matter how good the effects may be. A good effect indeed was produced (the surrender), but the means to achieve it were evil. Therefore, they are not justifiable. Evil effects were willed by means of inherently evil acts.
I freely grant the good intentions and good faith of those who disagree with me on this. But I cannot agree with their moral logic. In my opinion, it violates traditional just war criteria, the specific principle of double effect, and also the standards of the larger natural law, whereby it is instinctively known by virtually everyone that deliberate killing of women and children (in wartime as well as outside of it) is inherently wrong.
It's been argued that one might obliterate the civilian / soldier distinction in a militaristic society like wartime Japan, with its suicides and maniacal tendencies, etc. But how could a three-month-old baby be part of that, or a senile old woman, or a mentally ill man, etc.? Even if we were to grant that (which would take some doing), there would still be many exceptions to the rule. The bombings, therefore, would still have to be judged as immoral, since they involved the killing (and I am not averse to using the word "murder" after having established the ethical reasoning chain above) of many "innocents" in terms of the war.
This is a far different scenario from a smart bomb taking out a munitions factory (the intent), with part of the shell skidding off to a house a mile away and burning it and its inhabitants (not the intent at all). That is tragic but justifiable. Dropping a nuclear bomb into the center of a heavily-populated city is not.