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.
Army of the Han dynasty
In 71 BC, Chang Hui and two other generals led a force of 100,000 to aid the Wusun against the Xiongnu. The majority of the forces failed to find any Xiongnu, but Chang Hui successfully aided the Wusun in defeating a Xiongnu invasion. However the Xiongnu came back in winter and took many captives. On the way back across the Altai Mountains, the Xiongnu suffered heavy casualties from a sudden blizzard, devastating their army. The next year the Xiongnu were attacked on all sides by Wusun, Wuhuan, and the Han. One third of all Xiongnu died.