As Jirim League was close to China proper, it was subjected to an enormous population pressure from the Chinese heartland. The Han immigrants came under the administration of Chinese prefectures, and the Mongol banner quickly shrunk. The banner became too small for the Mongols to keep nomadic pastoralism. From the 19th century on, they were forced to move northward or were settled into agricultural villages. In its final years, the Qing dynasty promoted Han colonization of Mongolia to increase tax revenues and build a borderland defense. The Republic of China, which overthrew the Qing dynasty, boosted Han colonization. By the 1920s, the most part of Darkhan Banner was controlled by ethnic Han.