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
Due to the many losses inflicted on the Xiongnu, rebellion soon broke out and former enslaved people rose up in arms. Around 80 BC, the Xiongnu attacked the Wusun in a punitive campaign and soon the Wusun monarch requested military support from the Han empire. In 72 BC, the joint forces of the Wusun and Han invaded the territory of the Luli King of the Right. Around 40,000 Xiongnu people and many of their livestock were captured before their city was sacked after the battle. The very next year, various tribes invaded and raided the Xiongnu territory from all fronts; Wusun from the west, Dingling from the north, and Wuhuan from the east. The Han forces had set out in five columns and invaded from the south. According to "Hanshu", this event marks the beginning of Xiongnu decline and the dismantlement of the confederation.