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.
Xiongnu
The Han dynasty made preparations for war when the Han Emperor Wu dispatched the explorer Zhang Qian to explore the mysterious kingdoms to the west and to form an alliance with the Yuezhi people in order to combat the Xiongnu. During this time Zhang married a Xiongnu wife, who bore him a son, and gained the trust of the Xiongnu leader. While Zhang Qian did not succeed in this mission, his reports of the west provided even greater incentive to counter the Xiongnu hold on westward routes out of China, and the Chinese prepared to mount a large scale attack using the Northern Silk Road to move men and material.