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 founder of the Xiongnu empire was Toumen. He was succeeded violently by his son Modu Shanyu, who then conquered and unified various tribes. At the peak of its power, the Xiongnu confederacy stretched from Lake Baikal in the north to the Great Wall in the south and from the Tian Shan mountains in the west to the Greater Khingan ranges in the east. In the 2nd century BC the Xiongnu turned their attention westward to the region of the Altai Mountains and Lake Balkash, inhabited by Indo-European-speaking nomadic peoples, including Yuezhi (Yüeh-chih in Wade–Giles), who had relocated from China's present-day Gansu Province as a result of their earlier defeat by the Xiongnu. Endemic warfare between these two nomadic peoples reached a climax in the latter part of the 3rd century and the early decades of the 2nd century BC; the Xiongnu were triumphant. The Yuezhi then migrated to the southwest where, early in the 2nd century, they began to appear in the Oxus (the modern Amu Darya) Valley, to change the course of history in Bactria, Iran, and eventually India.