[131]
Although Truman was told briefly on the afternoon of April 12 that he had a new, highly destructive weapon, it was not until April 25 that Secretary of War Henry Stimson told him the details. [132]

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.— Harry Truman, writing about the atomic bomb in his diary[133] on July 25, 1945[134]
Truman journeyed to Berlin for the Potsdam Conference with Joseph Stalin and the British leader Winston Churchill. He was there when he learned the Trinity test—the first atomic bomb—on July 16 had been successful. He hinted to Stalin that he was about to use a new kind of weapon against the Japanese. Though this was the first time the Soviets had been officially given information about the atomic bomb, Stalin was already aware of the bomb project—having learned about it through atomic espionage long before Truman did.[135][136][137]
In August, the Japanese government refused surrender demands as specifically outlined in the Potsdam Declaration. 