Which country declared war on Japan on August 8, 1945, just before V-J Day?

The Soviet Union declared war on Japan on August 8, 1945, just before V-J Day.

The declaration followed secret Allied agreements under which the Soviet Union had promised to enter the war against Japan after Germany's defeat. Soviet forces began invading Japanese-held Manchuria shortly after midnight on August 9, 1945. The offensive rapidly challenged Japan's position in northeast Asia.

The Soviet declaration came after the atomic bombing of Hiroshima on August 6 and immediately before the atomic bombing of Nagasaki on August 9. Historians continue to debate the relative importance of the atomic bombings and the Soviet entry in Japan's decision to surrender, but both developments transformed the strategic situation.

Japan's surrender was announced by Hirohito on August 15 and formally signed on September 2. The Soviet Union was represented at the signing ceremony and became one of the Allied signatories.