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.
Modu Chanyu
Modu ruled from 209 BCE to 174 BCE. He was a military leader under his father Touman, and later Chanyu of the Xiongnu, centred in modern-day Mongolia. He secured the throne and established a powerful Xiongnu Empire by successfully unifying the tribes of the Mongolian-Manchurian grassland in response to the crisis of the loss of Xiongnu pasture lands to invading Qin forces commanded by Meng Tian in 215 BCE. While Modu rode and then furthered the wave of militarization and effectively centralized Xiongnu power, the Qin quickly fell into disarray with the death of the first emperor in 210, leaving Modu a free hand to expand his Xiongnu Empire into one of the largest of his time. The eastern border stretched as far as the Liao River, the western borders of the empire reached the Pamir Mountains, whilst the northern border reached Lake Baikal.