The Soviet Union declared war on Japan on 8 August 1945 and began its invasion the next day.
Soviet Foreign Minister Vyacheslav Molotov informed Japan’s ambassador in Moscow that the Soviet government would consider itself at war from 9 August. Just after midnight on 9 August in the region, Soviet forces crossed into Japanese-held Manchuria and attacked on multiple fronts.
The Soviet entry was a major strategic shock to Japanese leaders. Tokyo had hoped that Moscow might mediate with the Western Allies, but the Soviet declaration ended that possibility. Soviet forces rapidly advanced through Manchuria, attacked Japanese positions in Korea, and launched operations against South Sakhalin and the Kuril Islands.
The declaration came between the atomic bombings of Hiroshima on 6 August and Nagasaki on 9 August. Historians continue to debate the relative weight of the Soviet invasion and the atomic bombings in Japan’s decision to surrender, but both events transformed the final days of the war.