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.
History of Mongolia
The founder of the Xiongnu empire was Toumen. He was succeeded violently by his son Modu Shanyu, who then conquered and unified various tribes. At the peak of its power, the Xiongnu confederacy stretched from Lake Baikal in the north to the Great Wall in the south and from the Tian Shan mountains in the west to the Greater Khingan ranges in the east. In the 2nd century BC the Xiongnu turned their attention westward to the region of the Altai Mountains and Lake Balkash, inhabited by Indo-European-speaking nomadic peoples, including Yuezhi (Yüeh-chih in Wade–Giles), who had relocated from China's present-day Gansu Province as a result of their earlier defeat by the Xiongnu. Endemic warfare between these two nomadic peoples reached a climax in the latter part of the 3rd century and the early decades of the 2nd century BC; the Xiongnu were triumphant. The Yuezhi then migrated to the southwest where, early in the 2nd century, they began to appear in the Oxus (the modern Amu Darya) Valley, to change the course of history in Bactria, Iran, and eventually India.