The United States wasn’t the only member of the Allied forces beginning to move against Japan in 1945. That year, with the fall of Axis powers in Europe, the Soviet Union began to turn its efforts toward the Pacific theater of the war. At the February 1945 Yalta conference, held with U.S. President Franklin D. Roosevelt, U.K. Prime Minister Winston Churchill, and Soviet leader Joseph Stalin, the Soviet Union agreed to join in the fight against Japan soon after the surrender of Germany.
More specifically, U.S. General Douglas MacArthur expected the Soviets to engage with Japanese forces in the occupied Chinese territory of Manchuria. Soon after that, the Americans would commence an invasion of the home islands of Japan. On August 8, 1945, the Soviet Union did just that, declaring war on Japan. Almost simultaneously, Soviet forces began their attack on Manchuria.
At that point, President Roosevelt had died and Harry Truman had taken his place. Truman met with Stalin and Churchill at the July 17-August 2, 1945 Potsdam Conference. There, after speaking with Truman, Stalin learned that the U.S. had a new and very powerful weapon on its hands, though the president didn’t necessarily specify that the U.S. had nuclear capabilities (Stalin almost certainly knew about the Manhattan Project anyway). That surely made Truman’s threat of “prompt and utter destruction” levied against Japan, per the Potsdam Declaration, sound like something far more serious than performative chest-thumping.
