Gyōza recipes are said to have reached Japan through returnees from Manchukuo, the Japanese-backed puppet state in Manchuria. After World War II, Japanese colonists and others returned from Northeast China, bringing familiarity with Chinese foods that influenced the development and spread of Japanese gyōza.
Manchukuo existed from 1932 until 1945 in territory occupied by Japan after the invasion of Manchuria. Japan installed Puyi, the last Qing emperor, as its nominal leader, but Japanese military authorities retained decisive control. Calling it a puppet state reflects that political reality.
The postwar culinary connection does not mean modern gyōza are simply unchanged Chinese jiaozi. Japanese gyōza developed recognizable traits, including thin wrappers, a strong garlic flavor, and a strong association with pan-frying. Returnees opened yatai and other food businesses, often selling gyōza beside ramen.
The alternatives do not fit this history. Taiwan and Korea were places connected to Japanese colonial rule and regional food exchange, while Mengjiang was a different Japanese-aligned regime in Inner Mongolia.