Which country declared war on Japan on August 8, 1945?

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

Soviet Foreign Minister Vyacheslav Molotov informed Japan’s ambassador in Moscow that the Soviet Union considered itself at war with Japan beginning August 9. Soviet forces then launched the invasion of Japanese-held Manchuria shortly after midnight on August 9 in local time.

The Soviet entry was a major development in the final phase of the Pacific War. Japan had hoped that Moscow might help mediate with the Western Allies, but the Soviet declaration ended that possibility. Soviet forces attacked Japanese positions across Manchuria and also later fought in southern Sakhalin and the Kuril Islands.

The declaration came three days after the atomic bombing of Hiroshima and one day before the bombing of Nagasaki. Japan announced its decision to accept the surrender terms on August 15, although the formal surrender document was signed on September 2.