On July 16 , the Trinity test shows that a plutonium bomb ( Fat Man ) is feasible and that a nuclear blast is even more powerful than scientists predicted . The uranium bomb ( Little Boy , which is untested but is expected to work ) leaves Los Alamos for Tinian island in the Pacific . At the Potsdam conference near Berlin , the Soviet leader Joseph Stalin promises to join the war against Japan . British prime minister Winston Churchill urges Truman to use the bomb so as to constrain Russian expansion , an argument seconded by Truman 's military advisers , who warn that unless Japan surrenders quickly it will have a Russian zone of occupation and the attendant problems
Trinity test
Trinity (nuclear test)
Hiroshima (film)