Roosevelt made the decision to make the atomic bomb on October 9, 1941.
As the Manhattan Project progressed Roosevelt followed developments with the assumption that the atomic bombs would be used "as made ready".
Roosevelt died and when Truman is briefed on the atomic bomb he decides to change nothing.
After the first two bombs are dropped on Japan and they indicate they will surrender, Truman makes the only decisionentirely his own about the atomic bomb during WW2 and orders the bombing stopped.
