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.
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.