When did the Soviet Union declare war on Japan in the final days of World War II?

The Soviet Union declared war on Japan on August 8, 1945.

The declaration took effect at midnight on August 9 in the Soviet Far East, when Soviet forces began invading Japanese-held Manchuria. This campaign is known as the Soviet–Japanese War and included attacks against Japanese positions in Manchuria, Korea, Sakhalin, and the Kuril Islands.

The Soviet entry followed the Yalta Conference commitment to enter the war against Japan after Germany’s defeat. Japan had hoped that Moscow might help mediate a negotiated settlement, but the Soviet declaration destroyed that diplomatic expectation.

The date is often confused with August 15, when Japan publicly announced acceptance of the Allied terms. The Soviet invasion began earlier and was one of the major developments surrounding Japan’s decision to surrender.