The Soviet Union was not informed officially of the Manhattan Project until Stalin was briefed at the Potsdam Conference on July 24 , 1945 , by U.S. President Harry S. Truman , eight days after the first successful test of a nuclear weapon . Despite their wartime military alliance , the United States and Britain had not trusted the Soviets enough to keep knowledge of the Manhattan Project safe from German spies : there were also concerns that , as an ally , the Soviet Union would request and expect to receive technical details of the new weapon
the first successful test of a nuclear weapon
Trinity (nuclear test)
Nuclear arms race