The Soviet Union declared war on Japan on 8 August 1945, entering the Pacific conflict days before Japan’s surrender announcement.
The declaration followed agreements made by the Allied leaders at the Yalta Conference. The Soviet government informed Japan that the Soviet-Japanese Neutrality Pact was no longer applicable and declared war late on 8 August. Soviet forces began major operations against Japanese forces in Manchuria shortly after midnight on 9 August.
The offensive, known as the Soviet invasion of Manchuria or the August Storm operation, rapidly defeated Japan’s Kwantung Army and also affected Japanese positions in Korea, Sakhalin, and the Kuril Islands. Japan had hoped that the Soviet Union might help mediate a negotiated settlement, so the declaration destroyed an important diplomatic assumption.
The Soviet entry occurred at nearly the same time as the atomic bombing of Nagasaki. Japan’s surrender resulted from several interacting pressures, including military defeats, blockade, bombing, the atomic attacks, and the Soviet offensive; historians disagree about the relative importance of each factor.