According to Henry Stimson, U.S. Secretary of War during World War II, why did the United States decide that it needed to use atomic bombs against Japan during the war?

According to Henry Stimson, U.S. Secretary of War during World War II, why did the United States decide that it needed to use atomic bombs against Japan during the war? What were the justifications? In contrast, why have modern historians argued the use of atomic weapons during World War II was unnecessary? Which side has the stronger argument and why?