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.
Han–Xiongnu War
The Southern Xiongnu—who had been situated in the Ordos region since about 50 AD—remained within the territory of the Han empire as semi-independent tributaries. They were dependent to the Han empire for their livelihood as indicated by a memorial from the Southern Chanyu to the Han court in 88 AD. Following the military successes against the Xiongnu, General Ban Chao was promoted to the position of Protector General and stationed in Kucha in 91 AD. At the remote frontier, Ban Chao reaffirmed absolute Han control over the Western Regions from 91 AD onwards.