Mrs D,
The indiscriminate killing of civilians is against the UCMJ et al. However, if civilians are killed in the conduct of otherwise lawful operations, there is no crime.
Destroying the military facilities in Hiroshima and Nagasaki, in addition to war plant, railroads, other lines of communication, all were bona fide military targets by the measure of the day. Each city was relatively untouched up to that point in the war. Absent precision weapons- a very recent innovation- the way to come close to attriting such targets was to attack a whole area.
Consider, for example, the relentless bombing attacks against Germany, where portions of large cities were flattened to take out a single factory. Bloodthirstiness was not foremost on the Allies' minds, but military effectiveness. And sending umpteen waves of bombers against a single area target was the only way, at the time, to effectively accomplish the mission.
Furthermore, and again considering the mentality of the time, "shielding" the Japanese population was probably not seriously considered nor, even if it were, was it plausible. Particularly considering the depth of that society's militarism, where every member was expected to fight to the death for the emperor. Whether that was a plausible scenario or not upon reflection, that was the generally understood situation at the time.
I think the core of our disagreement is whether there was sufficient military cause to justify destroying both cities, despite the civilian (ie, not direct uniformed combatant) cost of doing so. You must bear in mind that other munitions available might have caused even more death than the nukes did, and would surely have been used.
In terms of the immediate tactical problem, reasonably ensuring the destruction of whatever units and war plant were located there to support amphibious attack...possibly not.
In terms of the strategic problem of finishing the war by defeating the enemy, and defeat it such that there could be no doubt about victor and vanquished (and thereby preclude a post- WW1-style resurgence), I feel the atomic bombs were justified.
I don't take any pleasure from it, and I don't celebrate it. But I see their usage as a brutal conclusion to a brutal conflict.
I am in the rather difficult position of maintaining both that
I am not Truman's judge or Stimson's judge or Tibbets' judge. Bu I know Who it is who says that shedding innocent blood is an abomination --- and I know He judges rightly.