By the end of July the bomb was ready to use. Henry Stimson, the Secretary of War, sent Truman an urgent message:
The time schedule on Grove’s project [i.e. the bomb] is progressing so rapidly that it is now essential that statement for release by you [the press release to be issued after the bomb was dropped] be available not later than Wednesday, 1 August.
The President, who was at the Potsdam Conference in Germany, responded in a handwritten note on the back of Stimson’s letter. It is believed to be the only publicly available document in which Truman gives permission to drop the bomb:
Reply to your 41011
Suggestions approved
Release when ready
but not sooner than August 2.
HST
On the 24th of July, while at the Potsdam Conference, Truman told Joseph Stalin, the Soviet leader, that America had a "new weapon of unusual destructive force." With seemingly little interest, Stalin told the president he hoped the United States would make “good use of it against the Japanese.” Truman believed he had given Stalin new information. He was wrong. Through his spies, Stalin had known about the nuclear-weapon project since 1941.
The press release was soon finalized - except for the city where the bomb ("L-11") would be dropped. That most important detail was decided last.
It would be Hiroshima.