Hiroshima and the Elided Moral Question

Today is the 70th anniversary of the first deployment of a nuclear weapon against humans. Something that was followed three days later by the second and so far last such deployment. Which raises any number of moral questions. One is there something particularly immoral about nuclear warfare that does not apply to other methods, for example fire bombing of Tokyo and Dresden with conventional weapons? But rather than starting the debate from that perspective there seems to be an insistence that the use of the atomic bomb was entirely unnecessary and cruel which in turn rests on the assertion that Japan was defeated, knew it, and was prepared to surrender. But were they? And did the allies know that? If not the moral calculus starts from a totally different spot. My own position is that Hiroshima was necessary but that Nagasaki was a war crime, that the former served to save millions of lives on net while the latter was the result of cynical and cruel calculation and all about post-war positioning. Neither conclusion stemming from a starting point that has nuclear weapons being a moral evil in and of themselves. Instead the moral question then and now was whether dropping the first bomb ended the war or not. To me those who see Hiroshima as an act of evil have one challenge: show that it was more evil than best information alternatives. Until that is settled all else is hand waving.

I start from the perspective that War is a War Crime. That you can be forced into a Just War but you can never wage a Just War. As a popular poster said in my youth during the hottest stages of the Vietnam War “War is not Healthy for Little Girls. And Other Living Things.” A slightly different wording here:
War is not Healthy
Was World War II avoidable? If not the moral calculus involves bring the war to as just an end as possible while killing the fewest little girls. Did Hiroshima meet that test?