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
During the spring of 123 BC, General Wei Qing set off to Mongolia with an army to attack the Xiongnu; they marched back victorious to Dingxiang. Two months later, the Han army advanced towards the Xiongnu again, but this time the Xiongnu were prepared for the invasion by the Han forces. However, hereafter, due to the military expeditions that the Han empire undertook, the Xiongnu moved their capital and retreated to the far northern regions of the Gobi Desert.