I wasnt even alive then, and I think this is insanely too simple.

>If America was able to keep that secret

There is no America, there is the United States government. Americans did not want to join in World War 2, the president did.

When the time came to drop the nuclear bomb, these decisions were made by few people, and the popular theory is that the Democrats(incumbent) would have lost elections if the war went on, Americans died, and people found out about the Nuclear Bomb later.

The best decisions for the world are not made, the best decision for individuals are made.

I wonder what will happen when Putins or Xis back are against the wall politically. What horrors will they do for self preservation?

EDIT: Don't downvote because the Democrats were in power at the time. The Republicans would have done this too.