The Soviet Union invaded Japanese-held Manchuria on August 9, 1945.
The Soviet offensive began in the early hours of August 9 against Manchukuo, Japan's puppet state in Manchuria. Soviet forces attacked from several directions with large armored and mechanized formations, rapidly overwhelming the Japanese Kwantung Army and capturing major territory.
The Soviet declaration of war followed the commitments made at the Yalta Conference. The Soviet Union had agreed to enter the war against Japan within three months of Germany's defeat, and Germany surrendered in May 1945. The invasion therefore began almost exactly three months later.
The offensive was one of the major developments influencing Japan's decision to surrender. Historians debate the relative weight of the Soviet attack and the atomic bombings, but both sharply changed Japan's strategic situation. The Kwantung Army formally surrendered on August 16, although fighting continued in some areas until September.