As World War II dragged to an end in 1945, the leaders of the “Big Three” allied powers—the United States, Soviet Union, and Great Britain—met in Potsdam, Germany, to hash out  terms to conclude the bloodiest conflict the world had ever seen. The great powers split Germany into occupation zones, recognized a Soviet-backed government in Poland, and partitioned Vietnam, monumental decisions that shaped the postwar global order. The talks were meant to forge a lasting peace, but within 18 months, a Cold War began that lasted more than four decades.

One of the most important moments at Potsdam was not captured in a memo or proclaimed at a press conference. Late in the conference, U.S. President Harry Truman took aside Soviet