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.
Yuezhi
Gradually the Xiongnu grew stronger and war broke out between them and the Yuezhi. There were at least four wars between the Yuezhi and Xiongnu according to the Chinese accounts. The first war broke out during the reign of the Xiongnu monarch Touman (who died in 209 B.C) who suddenly attacked the Yuezhi. The Yuezhi wanted to kill Modu, the son of the Xiongnu king Touman kept as a hostage to them, but Modu stole a good horse from them and managed to escape to his country. He subsequently killed his father and became ruler of the Xiongnu. It appears that the Xiongnu did not defeat the Yuezhi in this first war. The second war took place in the 7th year of Modu era (203 B.C.). From this war, a large area of the territory originally belonging to the Yuezhi was seized by the Xiongnu and the hegemony of the Yuezhi started to shake. The third war probably was at 176 BC (or shortly earlier) and the Yuezhi were badly defeated.