At the Potsdam Conference in Germany , Truman was informed that the Trinity test had been successful . He told Stalin , the leader of the Soviet Union , that the US had a new superweapon , without giving any details . This was the first official communication to the Soviet Union about the bomb , but Stalin already knew about it from spies . With the authorization to use the bomb against Japan already given , no alternatives were considered after the Japanese rejection of the Potsdam Declaration
Potsdam Conference
Potsdam Conference
Manhattan Project