history : Why-did-the-US-choose-Japan-and-not-Germany-to-nuke -1
Answer by Jeremy White:
Five main reasons:
1) Germany had already surrendered
2) Japan wasn't going to surrender (so the theory goes) and an invasion would have cost significant Allied soldiers lives.
3) America wanted to get even after Pearl Harbor
4) Russia had entered the war with Japan and we needed an ending quickly to prevent Russia from gaining any influence.
5) We wanted to show Russia we had the bomb (and Germany had surrendered as mentioned).
Note- #1-2 are the usual reasons stated, while #3-5 are more controversial. I happen to believe the latter are substantial reasons but you will get people responding on here disagreeing with this.