Nomadic empire
The Xiongnu were a confederation of nomadic tribes from Central Asia with a ruling class of unknown origin and other subjugated tribes. They lived on the Mongolian Plateau between the 3rd century BC and the 460s AD, their territories including modern day Mongolia, southern Siberia, western Manchuria, and the modern Chinese provinces of Inner Mongolia, Gansu, and Xinjiang. The Xiongnu was the first unified empire of nomadic peoples. Relations between early Chinese dynasties and the Xiongnu were complicated and included military conflict, exchanges of tribute and trade, and marriage treaties. When Emperor Qin Shihuang drove them away from the south of the Yellow River, he built the famous Great Wall to prevent the Xiongnu from coming back.
Han–Xiongnu War
The first half of the 1st century BC witnessed several succession crises for the Xiongnu leadership, allowing the Han empire to reaffirm its control over the Western Regions. Huduershi Chanyu was succeeded by his son Punu (蒲奴) in 46 AD, thus breaking the late Huhanye's orders that only a Xiongnu ruler's brother was a valid successor. Bi (比), the Rizhu King of the Right and Huduershi's nephew, was outraged and was declared a rival Chanyu by eight southern Xiongnu tribes in 48 AD. The Xiongnu confederation fell apart in the Northern Xiongnu and Southern Xiongnu, and Bi submitted to the reign of the Han empire in 50 AD. The Han took control of the Southern Xiongnu under Bi, which had 30–40 thousand troops and a population of roughly twice or thrice the size.