In February 1945 at the Yalta Conference , the Soviet Union under Joseph Stalin agreed to enter the war against Japan within three months ' of Germany 's defeat . In order to meet this deadline , it was necessary that the Soviet Union and the Western Allies cooperate in stockpiling supplies in the Far East while the Red Army dispatched additional forces along the Trans-Siberian Railway . While the Japanese monitored this buildup , they did not believe the Soviets would be ready to attack until mid-September , resulting in their being taken by surprise when the attack actually began on August 8
Yalta Conference
Yalta Conference
Battle of Mutanchiang