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.
Wuhuan
In 209 BC, Modu Chanyu, founder of the Xiongnu empire, defeated the Donghu. The Wuhuan and Xianbei were remnants of the Donghu who had fled east. The Xianbei, of the lateral Donghu line, resided to the north of the Wuhuan. The Wuhuan were of the direct Donghu line and got involved in many relations with the Chinese due to their more southern location (around Liaodong). Until 121 BC, the Wuhuan was a tributary of the Xiongnu empire. The Hou Hanshu (Ch. 120) says: "From the time that Modu Shanyu crushed them the Wuhuan became weak. They were kept in constant subjugation to the Xiongnu and were forced to pay annual taxes of cow, horse and sheep skins. If anybody did not pay this tax his wife and children were taken from him."