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.
History of Mongolia
The first significant appearance of nomads came late in the 3rd century BC, when the Chinese repelled an invasion of the Xiongnu (Hsiung-nu in Wade–Giles romanisation) across the Yellow River from the Gobi. A Chinese army, which had adopted Xiongnu military technology—wearing trousers and using mounted archers with stirrups—pursued the Xiongnu across the Gobi in a ruthless punitive expedition. Fortification walls built by various Chinese warring states were connected to make a 2,300-kilometre Great Wall along the northern border, as a barrier to further nomadic inroads.