On August 9 , 1945 , Stalin , based on a secret agreement at the Yalta Conference in February , unilaterally abrogated the USSR 's Neutrality Treaty with Japan ( 1941 ) and declared war on Japan on August 9 , 1945 , beginning the Soviet – Japanese War . The Soviets invaded Manchuria on three fronts . The Potsdam Declaration was intended from the start to serve as legal basis for handling Japan after the war . Following the surrender of the Japanese government and the landing of General McArthur in Japan in September 1945 , the Potsdam Declaration served as legal basis for occupation reforms
Yalta Conference
Yalta Conference
Potsdam Declaration