Many prominent Americans thought it was wrong to drop the bombs on a beaten enemy at that time, among those people were just about every top American commander in the Pacific, including................

General Douglas MacArthur, was not consulted beforehand, but said afterward that there was no military justification for the bombings. The same opinion was expressed by Fleet Admiral William Leahy (the Chief of Staff to the President), General Carl Spaatz (commander of the U.S. Strategic Air Forces in the Pacific), and Brigadier General Carter Clarke (the military intelligence officer who prepared intercepted Japanese cables for U.S. officials) and also Major General Curtis LeMay and Admiral Ernest King, U.S. Chief of Naval Operations, and Fleet Admiral Chester W. Nimitz, Commander in Chief of the Pacific Fleet.

Plus General Dwight Eisenhower, expressing "grave misgivings" over Truman’s political decision to use the atomic bombs, notes in his memoirs:

'Japan was already defeated … dropping the bomb was completely unnecessary (and) no longer mandatory as a measure to save American lives. It was my belief that Japan was, at that very moment, seeking some way to surrender with a minimum loss of face'


Some critics believe that the U.S. had ulterior motives in dropping the bombs, including justifying the $2 billion investment in the Manhattan Project, testing the effects of nuclear weapons, exacting revenge for the attacks on Pearl Harbor, and demonstrating U.S. capabilities to the Soviet Union and the Americans wanted it finished before the Soviets had much say in the Pacific, but this was only after America had the bomb, until then the U.S. were eager to have Soviet help, which Stalin promised as soon as the Nazis were beaten.

On the need for invasion and up to a million casualties the US Strategic Bombing Survey reported....

'Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey's opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated'


But having said that, I remember someone from a Japanese occupied country once posted why have the Japanese killing or committing atrocities for even one more day, end the war quickly by any means possible.

I think he may have a point.