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.
Mongols
After the Donghu were defeated by Xiongnu king Modu Chanyu, the Xianbei and Wuhuan survived as the main remnants of the confederation. Tadun Khan of the Wuhuan (died 207 AD) was the ancestor of the proto-Mongolic Kumo Xi. The Wuhuan are of the direct Donghu royal line and the "New Book of Tang" says that in 209 BCE, Modu Chanyu defeated the Wuhuan instead of using the word Donghu. The Xianbei, however, were of the lateral Donghu line and had a somewhat separate identity, although they shared the same language with the Wuhuan. In 49 CE the Xianbei ruler Bianhe (Bayan Khan?) raided and defeated the Xiongnu, killing 2000, after having received generous gifts from Emperor Guangwu of Han. The Xianbei reached their peak under Tanshihuai Khan (reigned 156–181) who expanded the vast, but short lived, Xianbei state (93–234).