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.
Emperor Wen of Han
In 169 BC, Chao Cuo (晁錯), then a low-level official, offered Emperor Wen a number of suggestions for dealing with the Xiongnu. Emperor Wen was impressed, and made him a member of Crown Prince Qi's household. At Chao's suggestion, in 168 BC, Emperor Wen instituted the policy that if people contributed food for use by the northern defense force against Xiongnu, they could receive titles or have their crimes pardoned.