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
The Xiongnu also practiced marriage alliances with Han dynasty officers and officials who defected to their side. The older sister of the Chanyu (the Xiongnu ruler) was married to the Xiongnu General Zhao Xin, the Marquis of Xi who was serving the Han dynasty. The daughter of the Chanyu was married to the Han Chinese General Li Ling after he surrendered and defected. The Yenisei Kirghiz Khagans claimed descent from Li Ling. Another Han Chinese General who defected to the Xiongnu was Li Guangli who also married a daughter of the Chanyu.