Harry Truman and The Atomic Bomb As Vice-president, Harry Truman had not known about the development of the atomic bomb. On the day he assumed the Presidency at the death of Franklin D. Roosevelt, Secretary of War Henry L. Stimson had spoken to him briefly and told him that the United States was working on a weapon of extraordinary power. Twelve days later, on April 25, 1945, Stimson and Maj. Gen. Leslie R. Groves, director of the Manhattan Project, briefed President Truman in detail on the secret of the atomic bomb. The bomb had not yet been tested. Once it was proved to work, Truman would cons