The death of Roosevelt had catapulted Harry S. Truman into a position of awesome responsibility for which he had little preparation. He had not been privy to the work of the Atomic research programme while Vice President but now he had to give the final authorisation for the use of the new weapons.
The Allies were still actively planning Operation Downfall, the invasion of the Japanese mainland, and their forecasts of anticipated casualties varied widely. In June President Truman had been briefed that it might cost over 250,000 US fatalities. Another study, after the bloody experience on Okinawa had been absorbed into the thinking, placed the range at up to 4 million U.S. casualties with fatalities between 400,000 – 800,000 – Japanese casualties were assumed to be several times greater, including a high proportion of civilians.
It was in this context that Truman was presented with the news of the successful Trinity test and the option of using the new Atomic Bomb. He outlined the circumstances in his diary, in which, curiously, he refers to himself as the US President:
Diary: Potsdam 25 July 1945
We met at eleven today. That is Stalin, Churchill, and the US President. But I had a most important session with Lord Mountbatten and General Marshall before that. We have discovered the most terrible bomb in the history of the world. It may be the fire destruction prophesied in the Euphrates Valley Era, after Noah and his fabulous Ark.
Anyway we “think” we have found the way to cause a disintegration of the atom. An experiment in the New Mexican desert was startling — to put it mildly. Thirteen pounds of the explosive caused the complete disintegration of a steel tower 60 feet high, created a crater 6 feet deep and 1,200 feet in diameter, knocked over a steel tower 1/2 mile away and knocked men down 10,000 yards away. The explosion was visible for more than 200 miles and audible for 40 miles and more.
This weapon is to be used against Japan between now and August 10th. I have told the Sec. of War, Mr Stimson, to use it so that military objectives and soldiers and sailors are the target and not women and children. Even if the Japs are savages, ruthless, merciless and fanatic, we as the leader of the world for the common welfare cannot drop this terrible bomb on the old capital or the new.
He and I are in accord. The target will be a purely military one and we will issue a warning statement asking the Japs to surrender and save lives. I’m sure they will not do that, but we will have given them the chance.
It is certainly a good thing for the world that Hitler’s crowd or Stalin’s did not discover this atomic bomb. It seems to be the most terrible thing ever discovered, but it can be made the most useful.
On the 26th July the Allies issued a statement calling upon Japan to surrender or face “prompt and utter destruction”.