He authorized its development by the US.
FDR
FDR never saw a working atomic bomb, he died before the first ones were built.
FDR ordered its development.Truman ordered its use.
It is thought that he would have nixed the actual usage.
FDR
Not really.
Not at all.
No, he had no involvement whatsoever with the project. Einstein was a pacifist and refused to work on any war related project himself. His only involvement was to sign a letter written by Leo Szilard (the inventor of the atomic bomb) that was delivered to FDR warning of the danger if the Nazis built it and recommending starting a project.
FDR did not live long enough to order the atomic bombing of Japan having passed away April 12, 1945 .
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.
About the time Leo Szilard told him about it and had Einstein sign the letter Szilard had written to FDR.
Leo Szilard, the inventor of the atomic bomb (patent GB630726), did not build it himself. He wrote a letter that he had Albert Einstein sign and send to president FDR warning of the potential of atomic bombs and that the Nazis were probably working on them already. If the Nazis got atomic bombs first the Allied cause was lost. FDR authorized funds for a study, the study recommended a full program be begun under direction of the Army. This program became the Manhattan Project, which built the bomb. Leo Szilard was one of hundreds of scientists employed by the Manhattan Project during WW2.