Many believe the US used the both to scare the Soviets.
But the Soviets were probably the real reason Japan surrendered. New evidence, finally unearthed after six decades, indicates the Japanese wanted to avoid Soviet troops dissecting their island as they had already divided Germany. The Bomb may have had little to do with their submission.
Which is a tremendous multiple irony.
For years Franklin Roosevelt lobbied Soviet dictator Josef Stalin to enter the war against Japan. FDR did not want to go it alone in a land invasion.
Stalin had his hands full with Hitler. And after beating the Germans, his country was decimated.
Stalin was not eager for more expensive warfare against the Japanese, with whom he had maintained an uneasy neutrality.
But Roosevelt died in April, 1945. Relations between the US and USSR deteriorated. Harry Truman was far more hostile to the Soviets than FDR had been.
And he was willing to use the Bomb to intimidate Stalin or so he thought.
Stalin’s spy network had already made him well aware of the Bomb and what it could do. Nor is there reason to believe he ever doubted Truman would use it.
By August, 1945, Truman was far less eager to have the Soviets marching into Japan than FDR had been. Victory seemed certain. The Americans were not keen to share an occupation with the Russians, as they had to do in Germany.
The immediate American rationale for using the Bomb was that it would avoid the need for a land invasion of Japan. The much-publicized estimated cost of a million American lives was at best a guess, based on no hard numbers.
In any event, the US was in no position to invade at least until November. With the Russians coming from the east, Japan faced an inescapable vice.
So why August 6, and then August 9?
First was a desire that the Japanese surrender BEFORE the Soviets could get there.
Second was a desire to show Japan, the Soviets and the world that the US had this weapon, and was willing to use it.
Third, and most plausible: $2 billion had been spent to develop these weapons. Jimmy Byrnes, Truman’s Karl Rove of the day, warned that if they weren’t used, Congress and the American public would demand to know where all that money went.