What conflict began when the Soviet Union declared war on Japan on August 8, 1945?

The conflict that began when the Soviet Union declared war on Japan on August 8, 1945, was the Soviet–Japanese War. Soviet forces launched their major offensive against Japanese-held territory shortly after midnight on August 9.

The campaign focused on Manchuria, where Japan maintained the puppet state of Manchukuo, but fighting also spread to southern Sakhalin and the Kuril Islands. The Soviet offensive used forces from several directions and rapidly overwhelmed the Japanese Kwantung Army, which had once been one of Japan’s most powerful formations.

Japan’s leaders had hoped that the Soviet Union might mediate between Japan and the Western Allies. Instead, the Soviet declaration removed that diplomatic possibility and opened a major front in northeast Asia. The Soviet attack occurred at the same time as the Nagasaki bombing, contributing to the Japanese leadership’s decision to accept surrender terms.