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
During the next century, as Chinese strength waned, border warfare between the Chinese and the Xiongnu was almost incessant. Gradually the nomads forced their way back into Gansu and the northern part of what is now China's Xinjiang. In about the middle of the first century CE, a revitalized Eastern Han (25-220 CE) slowly recovered these territories, driving the Xiongnu back into the Altai Mountains and the steppes north of the Gobi. During the late first century, having reestablished the administrative control over southern China and northern Vietnam that had been lost briefly at beginning of this same century, the Eastern Han made a concerted effort to reassert dominance over Inner Asia.