This is false. There has never been any historical record at that time that shows that “warning the soviets” was any part of it. In fact, the USA disclosed the bomb to stalin beforehand hand. Anyone who postulated that was not in the chain of command. The Americans still very much believed at this point that the best post wore outcome was an ongoing alliance between the United States and the USSR. It was only after the scale of Stalin‘s ambitions in Eastern Europe became apparent and the aftermath of the Nuremberg trials that it was becoming obvious that there would be an ongoing confrontation.

I strongly suggest the recent book on operation downfall, as well as the recent popular works on the last months of the war from the Japanese point of view.