The Soviet Union invaded Manchuria when it entered the war against Japan in August 1945.
The Soviet offensive began just after midnight on August 9, 1945, against the Japanese-controlled puppet state of Manchukuo. Soviet forces attacked the Kwantung Army across a huge front, advancing through Manchuria with armored, mechanized, and airborne units.
The invasion fulfilled a promise made by Joseph Stalin to the Western Allies at the Yalta Conference. The Soviet Union declared war on Japan three months after Germany’s surrender, as agreed. Japanese leaders had hoped Moscow might mediate a negotiated settlement, so the Soviet attack destroyed an important diplomatic assumption.
Soviet forces also moved into parts of northern Korea, southern Sakhalin, and the Kuril Islands. The Kwantung Army officially surrendered on August 16, although fighting in the region continued. Historians debate the relative importance of the Soviet entry and the atomic bombings in Japan’s decision to surrender, but both events sharply changed the strategic situation in August 1945.