WHO made the decision to drop the atomic bomb on Japan?

Answer 1

Franklin D. Roosevelt made the decision in the last couple of months of his life, since Italy had surrendered and Germany was largely defeated that Japan would be the target if they continued to fight as strongly as it seemed they were. He had orders written up for the preparation of an initial list of target cities in Japan and planning for getting the bombs after Los Alamos built them to the pacific theater and dropped on those cities. But then he died and Harry S. Truman (who had been kept entirely in the dark) had to take over, he saw no reason to change any of these preparations, so the process continued as previously ordered to "use the atomic bombs as they became available".

After the bombing of Hiroshima and Nagasaki, when the Japanese government signalled their willingness to surrender Harry S. Truman made his one wartime decision on the use of the atomic bomb: stop dropping them on Japan.

Following the end of the war, to maintain a balanced federal budget, Harry S. Truman decided to totally gut the conventional military and depend 100% on strategic atomic bombing for any future wars. The thing he failed to realize was that his gutting of the conventional military also crippled our ability to deliver atomic bombs and hindered the Manhattan Project (and later the Atomic Energy Commision, when it took over) in manufacturing atomic bombs and training the teams that would be needed to assemble and arm the atomic bombs should they be needed for war.

Answer 2

President Truman