Soviet forces invaded Manchukuo on 9 August 1945 before Japan’s formal surrender.
Manchukuo was a Japanese-created puppet state in Manchuria, established after Japan’s occupation of the region in 1931. It was nominally ruled by Puyi, the former Qing emperor, but Japan controlled its political and military affairs. The Soviet offensive against it formed the central part of the Soviet invasion of Manchuria.
The Red Army attacked from several directions shortly after the Soviet Union declared war on Japan. Soviet forces advanced through Manchuria, Inner Mongolia, and northern Korea, rapidly overwhelming the Japanese Kwantung Army. Japan had hoped that the Soviet Union might mediate with the Western Allies, so the Soviet attack sharply changed the diplomatic and military situation.
The invasion began on 9 August, the same day the United States dropped an atomic bomb on Nagasaki. Japan’s leaders considered both developments alongside the Hiroshima bombing and the worsening military position. The Kwantung Army formally surrendered on 16 August, although fighting continued in some areas until the September signing ceremony.