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.
Xiongnu
Under Modu's leadership, the Xiongnu threatened the Han dynasty, almost causing Emperor Gaozu, the first Han emperor, to lose his throne in 200 BC. By the time of Modu's death in 174 BC, the Xiongnu had driven the Yuezhi from the Hexi Corridor, killing the Yuezhi king in the process and asserting their presence in the Western Regions.