The Soviet invasion of Japanese-held Manchuria began on August 9, 1945.
The Red Army attacked Manchukuo, Japan's puppet state in Manchuria, in the early hours of August 9. Soviet forces also moved against Japanese positions in Inner Mongolia and northern Korea. The operation is often called the Manchurian Strategic Offensive Operation or Operation August Storm.
The timing was crucial. The invasion began three days after the atomic bombing of Hiroshima and shortly before the bombing of Nagasaki on the same day. Japan had hoped the Soviet Union might help negotiate an end to the war, but the Soviet declaration of war destroyed that strategy.
The Kwantung Army had once been considered Japan's elite force, but it was understrength and poorly prepared in August 1945. Its rapid defeat, combined with the atomic bombings and the worsening military situation, is widely regarded as an important factor in Japan's decision to surrender.