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.
Military history of China before 1911
Armies during the Qin and Han dynasties largely inherited their institutions from the earlier Warring States Period, with the major exception that cavalry forces were becoming more and more important, due to the threat of the Xiongnu. Under Emperor Wu of Han, the Chinese launched a series of massive cavalry expeditions against the Xiongnu, defeating them and conquering much of what is now Northern China, Western China, Mongolia, Central Asia, and Korea. After these victories, Chinese armies were tasked with the goal of holding the new territories against incursions and revolts by peoples such as the Qiang, Xianbei and Xiongnu who had come under Chinese rule.