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.
Modu Chanyu
Modu ruled from 209 BCE to 174 BCE. He was a military leader under his father Touman, and later Chanyu of the Xiongnu, centred in modern-day Mongolia. He secured the throne and established a powerful Xiongnu Empire by successfully unifying the tribes of the Mongolian-Manchurian grassland in response to the crisis of the loss of Xiongnu pasture lands to invading Qin forces commanded by Meng Tian in 215 BCE. While Modu rode and then furthered the wave of militarization and effectively centralized Xiongnu power, the Qin quickly fell into disarray with the death of the first emperor in 210, leaving Modu a free hand to expand his Xiongnu Empire into one of the largest of his time. The eastern border stretched as far as the Liao River, the western borders of the empire reached the Pamir Mountains, whilst the northern border reached Lake Baikal.