Answer:
After the 1928 liberation, Beijing became free up to the second Japanese Sino war that began in 1937; it was elected as the sitting government in China and ruled as a puppet government that was occupied by Japanese.
In 1937, all-out Japanese in China launched an invasion, and in 1938, the Japanese had already controlled a large part of China. They then forced complicated alliance among the Jiang’s and Mao’s and put on hold their differences temporarily and focused on uniting the Japanese.