I've read opinions/theories that suggest the US didn't really need to bomb Hiroshima or Nagasaki, and that Japan would have surrendered soon enough, due to fears of a Soviet invasion, without that invasion needing to actually happen. The bomb drops were so the US could claim the achievement of getting Japan to surrender, which would give it prestige and leverage over the Soviets, and more of a say in what happened to Japan and the Pacific theatre after the war. (Which, if true, worked exactly as planned.)
Beyond that, there are moral questions about the use of nuclear weapons. I know people who think it would have been more moral to force a Japanese surrender via many Japanese and American soldiers to die during an invasion of Japan, than for the US to kill civilians with nuclear weapons. I'm not sure I agree with that line of thinking, but I can't dismiss it either.