Military history of the Three Kingdoms
By the time of the Three Kingdoms period the Xiongnu had already ceased to be a major power. Their last stand as a steppe empire was in 89 when Dou Xian led an army of 45,000 and decisively defeated them at the Battle of the Altai Mountains. The Xiongnu Empire was replaced by a loose confederation of disorganized Xianbei tribes, who were nonetheless highly aggressive and proved problematic to their southern neighbors. A southern branch of the Xiongnu split off from their northern neighbors in 50 AD and continued to exist as a Han puppet state in Bing Province. However, in 188 the Xiuchuge clan rebelled against their Chanyu, Qiangqu, for sending troops to aid the Han against the rebels Zhang Ju and Zhang Chun. They replaced Qiangqu with his son Yufuluo, who's hold on power was tenuous, and he was ousted over a marquis of the Xubu clan. When the marquis of Xubu died the next year an elderly king became the nominal head of state without the title of Chanyu and the Southern Xiongnu ceased to exist as a coherent entity. In 216 the Xiongnu of the Ordos were settled but the region remained outside the jurisdiction of Cao Wei's administration.
Proto-Mongols
The Donghu (or Tung Hu, the Eastern Hu), a proto-Mongol and/or Tunguz group mentioned in Chinese histories as existing as early as the 4th century BC. The language of the Donghu, unlike that of the Xiongnu, is believed by modern scholars to be proto-Mongolic. The Donghu were among the first peoples conquered by the Xiongnu. By the 1st century AD, the Donghu had split, along geographical lines in two: the proto-Mongolic Xianbei (Wade–Giles "Hsien-pei") in the north and the Wuhuan in the south. After the Xiongnu were driven back into their homeland by the Chinese (48 AD), the Xianbei (in particular) began moving (from apparently the north or northwest) into the region vacated by the Xiongnu.