Which Japanese puppet state did Soviet forces invade in August 1945?

Soviet forces invaded Manchukuo in August 1945. Manchukuo was Japan’s puppet state in Manchuria, established after Japan seized the region in 1931 and installed the last Qing emperor, Puyi, as its nominal ruler.

The Soviet invasion began just after midnight on August 9, following the Soviet declaration of war against Japan on August 8. Red Army forces attacked across several fronts and rapidly overwhelmed the Japanese Kwantung Army, which had once been one of Japan’s most powerful military formations.

The campaign occurred between the atomic bombings of Hiroshima and Nagasaki. Historians continue to debate the relative importance of the Soviet entry and the atomic attacks in Japan’s decision to surrender, but the invasion clearly removed Tokyo’s hope that the Soviet Union might mediate a favorable peace.

Manchukuo should not be confused with Mengjiang, another Japanese-backed state in Inner Mongolia. Soviet forces also entered areas beyond Manchukuo, including northern Korea and southern Sakhalin, during the wider Soviet–Japanese War.