"It was still viewed as wrong to kill civilians just for kicks, but if civilians were killed in the process of destroying an enemy's warmaking ability, that was to be expected."
The destruction of Hiroshima and Nagasaki were effectively a terror attack. It was a case of "Look how many civilians we can kill in one go. Now surrender." . It also served as a display of power to the Soviet Union, which is even less of a valid reason to kill over one hundred thousand civilians than seeking a surrender. If either Germany or Japan had dropped an atomic bomb on a largely civilian target, it would have been considered a war crime, not just now, but even then. A number of US officials at the time thought it was criminal too. e.g. Admiral William Leahy
Once it had been tested, President Truman faced the decision as to whether to use it. He did not like the idea, but he was persuaded that it would shorten the war against Japan and save American lives. It is my opinion that the use of this barbarous weapon at Hiroshima and Nagasaki was of no material assistance in our war against Japan. The Japanese were already defeated and ready to surrender because of the effective sea blockade and the successful bombing with conventional weapons... My own feeling was that in being the first to use it, we had adopted an ethical standard common to the barbarians of the Dark Ages. I was not taught to make wars in that fashion, and that wars cannot be won by destroying women and children.