The Soviet Union declared war on Japan on 8 August 1945, entering the Pacific War days before Japan surrendered. Soviet forces began attacking Japanese-held territory shortly after midnight on 9 August.
The declaration fulfilled a commitment made by Soviet leader Joseph Stalin to enter the war against Japan after Germany’s defeat. The Soviet offensive struck Japanese forces in Manchuria, Korea, southern Sakhalin, and the Kuril Islands. The largest target was the Japanese-controlled state of Manchukuo, where the Kwantung Army had been stationed.
The Soviet entry profoundly changed Japan’s strategic situation. Japanese leaders had hoped the Soviet Union might mediate between Japan and the Western Allies, but the declaration destroyed that possibility and opened another front. Historians continue to debate the relative importance of the Soviet offensive and the atomic bombings in Japan’s decision to surrender. Both developments occurred within a short period before Hirohito’s 15 August announcement.