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.
Han–Xiongnu War
During the Warring States period, the Qin, Zhao, and Yan states conquered various nomadic territories inhabited by the Xiongnu and other Hu peoples. They strengthen their new frontiers with elongated wall fortifications. By 221 BC, the Qin ended the chaotic Eastern Zhou period by conquering all other states and unifying the entire nation. In 215 BC, Qin Shi Huang ordered General Meng Tian to set out against the Xiongnu tribes, situated in the Ordos region, and establish a frontier region at the Ordos Loop. Believing that the Xiongnu were a possible threat, the emperor launched a pre-emptive strike against the Xiongnu with the intention to expand his empire. Later that year (215 BC), General Meng Tian succeeded in defeating the Xiongnu and driving them from the Ordos region, seizing their territory as result. After the catastrophic defeat at the hands of Meng, Touman Chanyu and his followers fled far into the Mongolian Plateau. Fusu (Prince of Qin) and General Meng Tian were stationed at a garrison in Suide and soon began with the construction of the walled defenses, connecting it with the old walls built by Qin, Yan and Zhao states. The fortified walls ran from Liaodong to Lintao, thus enclosing the conquered Ordos region, safeguarding the Qin empire against the Xiongnu and other northern nomadic people. Due to the northward expansion, the threat that the Qin empire posed to the Xiongnu ultimately led to the state formation of the many tribes towards a confederacy.