Here's the million dollar question. Would the US have dropped the A-Bomb on a German city if the Nazis still had a capable fight force, and had not already surrendered?
In order to get a nations soldiers to justify killing en masse, without regret, you have to dehumanize your enemy. In the Japanese-American dynamic that was certainly the case (used by both sides); but interestingly enough, not used with the German-American one.
I agree the A-Bomb ended the war, and saved millions of lives, but would have it ever been considered.... seriously, to be deployed against a "European" nation? I think not, boots would have been put on the beaches.
Actually, one of the main reasons for dropping the bomb on a Japanese city was the US was fearful of reverse engineering. The US viewed the Japanese as subhuman opposed to Europeans which would hit a little closer to home per say.
In the event that the bomb didn't detonate as expected, the Japanese would be less likely to use the technology effectively against us.