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.
Su Wu
In 100 BC, there was a short-lived détente between long-term adversaries the Han and the Xiongnu. One year earlier, in 101 BC, a new chanyu had come to power in Xiongnu, Chanyu Qiedihou (且鞮侯), who had expressed an interest in peace with the Han and who, as a goodwill gesture, had allowed some Han diplomats who had been detained by the Xiongnu to return to Han, along with gifts from the chanyu.