At the Tehran Conference in November 1943 , Joseph Stalin agreed that the Soviet Union would enter the war against Japan once Germany was defeated . At the Yalta Conference in February 1945 , Stalin agreed to Allied pleas to enter World War II 's Pacific Theater within three months of the end of the war in Europe . On July 26 , the US , UK and China made the Potsdam Declaration , an ultimatum calling for the Japanese surrender which if ignored would lead to their " prompt and utter destruction "
Yalta Conference
Yalta Conference
Soviet–Japanese War