The Russians agreed to enter the war against Japan 3 months after the defeat of Germany and indeed commenced operations on 8th August (between the two bombs) and I'm sure the Americans wanted Japan to be defeated before the Russians established any influence in the area. The invasion of Okinawa was so costly that the US balked at invading Japan itself. Morally, I'm in two minds about this. The firebombing of Tokyo killed hundreds of thousands and I struggle to see the difference between dropping one bomb or hundred. They all kill indiscriminately. The question we should be asking, I suppose, is whether Japan would have attacked Pearl Harbor if America had had nuclear bombs at the time.