The Soviet Union entered the war against Japan on 8 August 1945.
The Soviet government declared war on Japan late on 8 August, taking effect in the early hours of 9 August in some accounts. Soviet forces then launched a massive offensive against Japan’s Kwantung Army in Manchuria, along with operations in other Japanese-held territories.
Japan had hoped the Soviet Union might mediate between Tokyo and the Western Allies, even though the Soviet Union had secretly promised at Yalta to enter the Pacific War. Soviet intervention therefore shocked Japanese leaders and removed a possible diplomatic channel.
The Soviet attack occurred between the atomic bombings of Hiroshima and Nagasaki. Historians continue to debate the relative weight of the bombings and Soviet entry in Japan’s decision to surrender, but both events profoundly affected the leadership crisis. Japan announced its acceptance of the Potsdam Declaration on 15 August, and the formal surrender followed on 2 September.