Which country declared war on Japan on August 9, 1945, during the final surrender crisis?

The Soviet Union declared war on Japan on August 9, 1945, during the final surrender crisis.

The declaration followed the Soviet Union’s decision to abandon its neutrality toward Japan. Soviet forces then launched an invasion of Japanese-held Manchuria, beginning the Soviet–Japanese War. The attack came between the American atomic bombings of Hiroshima on August 6 and Nagasaki on August 9.

Japan’s leaders had hoped that the Soviet Union might help mediate an end to the war. The Soviet attack destroyed that diplomatic possibility and opened a major front against Japan’s forces in northeast Asia. The Soviet Union also attacked Japanese positions in southern Sakhalin and the Kuril Islands.

Historians continue to debate the relative weight of the atomic bombings and Soviet entry in Japan’s decision to surrender. Both developments occurred within days of each other and placed extraordinary pressure on the Japanese government. Hirohito ordered acceptance of the Allied terms, leading to the August 15 announcement and September 2 signing.