The Soviet Union declared war on Japan on August 8, 1945, helping trigger Japan’s surrender crisis.
The declaration followed a secret Allied understanding made at the Yalta Conference. The Soviet Union had maintained a neutrality pact with Japan, but it renounced that arrangement and entered the war in the final weeks of World War II. Soviet forces began their invasion of Japanese-held Manchuria shortly afterward, on August 9 in the region’s local time.
The Soviet entry created a new strategic shock for Japanese leaders. Japan had hoped that Moscow might mediate a negotiated settlement, but the Soviet attack destroyed that possibility. The timing overlapped with the atomic bombing of Nagasaki on August 9. Historians continue to debate the relative weight of the Soviet invasion and the atomic bombings, but both events preceded Hirohito’s surrender announcement.