History of Mongolia
Although the Xiongnu finally had been split into two parts in AD 48, the Xianbei (or Hsien-pei in Wade–Giles) had moved (apparently from the east) into the region vacated by the Xiongnu. The Xianbei were the northern branch of the Donghu (or Tung Hu, the Eastern Hu), a proto-Mongol group mentioned in Chinese histories as existing as early as the 4th century BC. The language of the Donghu is believed to be proto-Mongolic to modern scholars. The Donghu were among the first peoples conquered by the Xiongnu. Once the Xiongnu state weakened, however, the Donghu rebelled. By the 1st century AD, two major subdivisions of the Donghu had developed: the proto-Mongolic Xianbei in the north and the Wuhuan in the south.
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.