I cannot remember the exact number but since the Japanese had so many islands as well as mainland Japan, the cost of invasion in lives (Western and Japanese) was massive. 
Of course it was not just a numbers decision, there was certainly some posturing for the post war world. 
The US was very concerned that Russia would not honor the agreements around splitting up Korea and roll on into Seoul. 
Then you had Germany and Eastern Europe to think about.<p>It is interesting to theorize what would&#x27;ve happened after the war if the U.S. had not developed the bomb or if the U.S. had developed it but chose not to use it.