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
Due to the many losses inflicted on the Xiongnu, rebellion soon broke out and former enslaved people rose up in arms. Around 80 BC, the Xiongnu attacked the Wusun in a punitive campaign and soon the Wusun monarch requested military support from the Han empire. In 72 BC, the joint forces of the Wusun and Han invaded the territory of the Luli King of the Right. Around 40,000 Xiongnu people and many of their livestock were captured before their city was sacked after the battle. The very next year, various tribes invaded and raided the Xiongnu territory from all fronts; Wusun from the west, Dingling from the north, and Wuhuan from the east. The Han forces had set out in five columns and invaded from the south. According to "Hanshu", this event marks the beginning of Xiongnu decline and the dismantlement of the confederation.