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.
Xiongnu
Barfield attempted to interpret Xiongnu history as well as narrate it. He made the following points: The Xiongnu confederation was unusually long-lived for a steppe empire. The purpose of raiding China was not simply for goods, but to force the Chinese to pay regular tribute. The power of the Xiongnu ruler was based on his control of Chinese tribute which he used to reward his supporters. The Han and Xiongnu empires rose at the same time because the Xiongnu state depended on Chinese tribute. A major Xiongnu weakness was the custom of lateral succession. If a dead ruler's son was not old enough to take command, power passed to the late ruler's brother. This worked in the first generation but could lead to civil war in the second generation. The first time this happened, in 60 BC, the weaker party adopted what Barfield calls the 'inner frontier strategy.' They moved south and submitted to China and then used Chinese resources to defeat the Northern Xiongnu and re-establish the empire. The second time this happened, about 47 AD, the strategy failed. The southern ruler was unable to defeat the northern ruler and the Xiongnu remained divided.