Which country declared war on Japan on August 8, 1945, then invaded Manchuria?

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

Soviet forces crossed into Japanese-held Manchuria shortly after midnight on August 9. The offensive overwhelmed the Japanese Kwantung Army and also affected Inner Mongolia, northern Korea, South Sakhalin, and the Kuril Islands. Mongolia participated alongside the Soviet Union, but the Soviet declaration and main offensive made the USSR the answer to this question.

Japan had hoped that the Soviet Union might mediate a conditional peace with the Western Allies. The Soviet attack destroyed that hope and opened a major front in northeast Asia. Historians continue to debate the relative importance of the Soviet invasion and the atomic bombings in Japan’s decision to surrender.