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.
Proto-Mongols
Between 130-121 BCE, Chinese armies drove the Xiongnu back across the Great Wall, weakening their hold on Gansu as well as on what is now Inner Mongolia, and finally pushed them north of the Gobi into central Mongolia. Following these victories, the Chinese expanded into the areas later known as Manchuria, Mongolia, the Korean Peninsula, and Inner Asia. The Xiongnu, once more turning their attention to the west and the southwest, raided deep into the Amu Darya valley between 73-44 BCE. The descendants of the Yuezhi and their Chinese rulers, however, formed a common front against the Xiongnu and repelled them.