President Truman knew that American intelligence had broken Japan's secret code and was aware of what the Japanese leaders were saying and doing. Truman was anxious to get Joseph Stalin, the leader of the Soviet Union, to enter the war against Japan and had previously received a promise from Stalin to do so after Germany's defeat, even though he had signed a non-aggression treaty with Japan early in the war. Truman wanted confirmation from Stalin that the Soviet Union would go to war against Japan. On July 24, 1945, as the Potsdam Conference entered its second week, Truman told Stalin about the weapon, though he did not mention it was an atomic bomb.

Truman's original initiatives for the Potsdam negotiations were all crafted under the assumption that the United States would not be able to use the bomb at all. Truman's desire to demonstrate the new capability of the American military was tempered by wartime insistence on secrecy. However, he was able to find a moment at the conference to vaguely mention the bomb to Stalin.

Sources:
https://www.crf-usa.org/bill-of-rights-in-action/bria-15-3-b-choices-truman-hirohito-and-the-atomic-bomb
https://www.washingtonpost.com/news/retropolis/wp/2018/07/17/he-is-honest-but-smart-as-hell-when-truman-met-stalin
https://ahf.nuclearmuseum.org/ahf/history/potsdam-crossroads-atomic-science-and-international-diplomacy
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