The Big Three meet a final time, and one of them already has the bomb
What happened
From 17 July to 2 August 1945, in the Berlin suburb of Potsdam, Truman, Churchill, and Stalin held the last wartime meeting of the Big Three, this time with Germany already defeated and occupied. Churchill was replaced partway through, on 26 July, by Clement Attlee, after Attlee's party won the British general election while the conference was still underway. On 24 July, Truman told Stalin the United States had a new weapon of unusual destructive force, a guarded reference to the Trinity test conducted eight days earlier; Stalin, through his own intelligence networks, likely already knew more about the American atomic program than Truman realized. On 26 July, the United States, Britain, and China, but not the Soviet Union, which had not yet declared war on Japan, issued the Potsdam Declaration, an ultimatum demanding Japan's immediate unconditional surrender and warning that refusal would bring prompt and utter destruction.
Why it matters
Potsdam was the moment the wartime alliance's unity visibly began to crack into what would become the Cold War: the same conference that finalized Germany's occupation zones also exposed deep disagreement over Poland's postwar borders and government. Truman's guarded disclosure to Stalin, made without knowing Soviet intelligence had already penetrated the Manhattan Project, meant the secret weapon reveal changed almost nothing about what Moscow already understood, even though Washington believed it had caught Stalin by surprise.
How we know
The US State Department's Office of the Historian publishes the Potsdam Conference record within its official Foreign Relations of the United States series, documenting Churchill's mid-conference replacement by Attlee, the exact terms of the Potsdam Declaration's ultimatum, and the date Truman informed Stalin of the successful atomic test.
Sources
- US Department of State, Office of the Historian. The Potsdam Conference, 1945 · Reputable sourcehistory.state.gov · The domain "history.state.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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