While Truman did not know the exact details of Stalin's discussions with Imperial Japanese leadership, Truman had hoped to get the Soviets to join in the U.S. war against Japan. The Soviet Union had drawn up detailed operations plan for a major landing, in return, Stalin expected Soviet control over certain territories annexed at the beginning of the war by Japan and Germany. However, Truman had refrained from informing the Soviet leader about the Manhattan Project, which had just successfully tested the world’s first atom bomb but knew that the new weapon strengthened his hand. 

And so, on Aug. 18, in order to prevent the scenario of the Soviet occupation of a large part of Japan post-war, Truman bluntly stopped Stalin's final go-ahead of a land invasion of Japan.  Stalin procrastinated, weighing the pros and cons, and two days before the planned Aug. 24 landing on Hokkaido, Stalin called off the operation.
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