The Soviet Union declared war on Japan on August 8, 1945, entering the Pacific War’s final phase.
The declaration followed an agreement made with the other major Allies that the Soviet Union would enter the war against Japan after Germany’s defeat. Soviet forces began invading Japanese-held Manchuria shortly after midnight on August 9, according to local timing.
The Soviet entry created a major strategic crisis for Japan. Japanese leaders had hoped that the Soviet Union might mediate between Japan and the Western Allies, but the invasion ended that possibility. The Soviet offensive also threatened Japanese positions in Manchuria, Korea, southern Sakhalin, and the Kuril Islands.
The declaration came between the atomic bombings of Hiroshima and Nagasaki. Historians continue to debate the relative importance of the Soviet invasion and the atomic bombs in Japan’s decision, but both events occurred immediately before the surrender announcement.