Japanese genocide vs. American genocide?

Many historians and people feel that dropping the atomic bombs on Japan was uneed to end the war and can be considered a mass murder of civilions. Maybe not genocide, but something close to the fact. I feel that how is this any different from the rape of Nanking? The Japanese army purposely tried to kill innocent civilians including woman and small children for the fact that they didnt want them to get in the way. There was no war at the time. Japan was just trying to get more land and these people happened to get in the way, so they killed them all. Basically a genocide. I feel that this is way worse then what we did to Japan by dropping the A-bombs. We were still in a war with Japan even if their defeat did seem evident. If you think that America did commit some kind of crime by bombing innocent Japanese cities, think about what Japan has done and even other countries have done in the past. There have been way worse things to happen other than the dropping of the A-bombs.