The Japanese-backed puppet state linked to recipes that developed into gyōza was Manchukuo.
Manchukuo was established by Imperial Japan in 1932 in Northeast China, following Japan’s seizure of Manchuria. It formally ended in 1945, when the Soviet Union invaded the region near the close of World War II. Japanese settlers and soldiers encountered local Chinese dumpling traditions there.
After the war, millions of Japanese civilians and soldiers returned from territories including Manchukuo. Some brought Chinese culinary techniques and recipes back to Japan. Repatriates and food-stall operators helped popularize gyōza, often serving it alongside ramen. Gyōza therefore has Chinese culinary roots but became distinctly Japanese in its postwar development.
A common mix-up is to treat gyōza as simply another name for a single Chinese dumpling. Japanese gyōza commonly feature a strong garlic flavor, pork-and-cabbage filling, and thin machine-made wrappers, while Chinese jiaozi vary widely by region and preparation method.