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.
Mongols
After the Donghu were defeated by Xiongnu king Modu Chanyu, the Xianbei and Wuhuan survived as the main remnants of the confederation. Tadun Khan of the Wuhuan (died 207 AD) was the ancestor of the proto-Mongolic Kumo Xi. The Wuhuan are of the direct Donghu royal line and the "New Book of Tang" says that in 209 BCE, Modu Chanyu defeated the Wuhuan instead of using the word Donghu. The Xianbei, however, were of the lateral Donghu line and had a somewhat separate identity, although they shared the same language with the Wuhuan. In 49 CE the Xianbei ruler Bianhe (Bayan Khan?) raided and defeated the Xiongnu, killing 2000, after having received generous gifts from Emperor Guangwu of Han. The Xianbei reached their peak under Tanshihuai Khan (reigned 156–181) who expanded the vast, but short lived, Xianbei state (93–234).