The US had more reasons to drop these bombs than just the war. For starters, they built it to drop it on Hitler's head, not Japan. Second, once it was completed they wanted to test the damn thing. Third they wanted to prove their point to the rest of the world, especially the USSR which hastily started developping and producing nuclear weapons too. Perhaps the worst thing about the bombs is that after the explosions, american researchers went to Japan to study the effects of radiation poisoning instead of treating them. Even worse, american newspapers reported to american citizens that the bombs killed a lot of people "cleanly" and that there was no radiation poisoning, even though the military knew the atrocities.