stylistic:style_transfer, information:classification, structural:reformatting
Republic of China and the Second World War periods

Outer Mongolia gained independence from the Qing dynasty in 1911, when the Jebtsundamba Khutugtu of the Khalkha was declared the Bogd Khan of Mongolia. Although almost all banners of Inner Mongolia recognised the Bogd Khan as the supreme ruler of Mongols, the internal strife within the region prevented a full reunification. The Mongol rebellions in Inner Mongolia were counterbalanced by princes who hoped to see a restored Qing dynasty in Manchuria and Mongolia, as they considered the theocratic rule of the Bogd Khan would be against their modernising objectives for Mongolia. Eventually, the newly formed Republic of China promised a new nation of five races (Han, Manchu, Mongol, Tibetan and Uyghur). and suppressed the Mongol rebellions in the area.

The Republic of China reorganised Inner Mongolia into provinces:

Some Republic of China maps still show this structure.

The history of Inner Mongolia during the Second World War is complicated, with Japanese invasion and different kinds of resistance movements. In 1931, Manchuria came under the control of the Japanese puppet state Manchukuo, taking some Mongol areas in the Manchurian provinces (i.e., Hulunbuir and Jirim leagues) along. Rehe was also incorporated into Manchukuo in 1933, taking Juu Uda and Josutu leagues along with it. These areas were occupied by Manchukuo until the end of World War II in 1945.

In 1937, the Empire of Japan openly and fully invaded the Republic of China. On 8 December 1937, Mongolian Prince Demchugdongrub (also known as "De Wang") declared independence for the remaining parts of Inner Mongolia (i.e., the Suiyuan and Chahar provinces) as Mengjiang, and signed agreements with Manchukuo and Japan. Its capital was established at Zhangbei (now in Hebei province), with the Japanese puppet government's control extending as far west as the Hohhot region. The Japanese advance was defeated by Hui Muslim General Ma Hongbin at the Battle of West Suiyuan and Battle of Wuyuan. Since 1945, Inner Mongolia has remained part of China.

The Mongol Ulanhu fought against the Japanese.

Ethnic Mongolian guerrilla units were created by the Kuomintang Nationalists to fight against the Japanese during the war in the late 30s and early 40s. These Mongol militias were created by the Ejine and Alashaa based commissioner's offices created by the Kuomintang. Prince Demchugdongrub's Mongols were targeted by Kuomintang Mongols to defect to the Republic of China. The Nationalists recruited 1,700 ethnic minority fighters in Inner Mongolia and created war zones in the Tumet Banner, Ulanchab League, and Ordos Yekejuu League.

The Inner Mongolian People's Republic was founded shortly after the Second World War. It existed from 9 September 1945 until 6 November 1945.