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.
Proto-Mongols
Between 130-121 BCE, Chinese armies drove the Xiongnu back across the Great Wall, weakening their hold on Gansu as well as on what is now Inner Mongolia, and finally pushed them north of the Gobi into central Mongolia. Following these victories, the Chinese expanded into the areas later known as Manchuria, Mongolia, the Korean Peninsula, and Inner Asia. The Xiongnu, once more turning their attention to the west and the southwest, raided deep into the Amu Darya valley between 73-44 BCE. The descendants of the Yuezhi and their Chinese rulers, however, formed a common front against the Xiongnu and repelled them.