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.
Origin of the Huns
The chief piece of evidence linking the Xiongnu to the European and Iranian Huns is the similarity of their names. Supporting evidence is provided by historical records indicating that the term "Xiongnu" was used for the people referred to in Sogdian and Sanskrit texts as the "Xwn" and "Huṇa" respectively, terms used for peoples called Huns in the West. Another important connection is the use of similar metallic cauldrons by the European Huns and the Xiongnu. There remain a number of outstanding differences, however, including generally different archaeological profiles and a wide variety of customs attested among the various Hunnic groups. Additionally, there is a two hundred year gap between the last recorded activities of the Xiongnu and the first appearance of the Huns in Europe and Central Asia. These issues have caused some scholars to reject the identification.