The Kumo Xi and the Khitans were united in a tribal Kumo Xi entity. In 388 AD, the previously defeated Kumo Xi leaders insisted on fighting against with the XianbeiNorthern Wei dynasty. When the battlefield seemed to favor Wei, the Khitans fled, leaving the Kumo Xi be crushed. The Kumo Xi were so badly weakened by their defeat to the Northern Wei that the Khitans were able to freely split from the Kumo Xi afterwards, thereby starting their own independent history. The Kumo Xi were descendants of the Wuhuan. The Hou Hanshu records that “the language and culture of the Xianbei are the same as the Wuhuan”. Along with the Xianbei, the Wuhuan formed part of the proto-MongolicDonghu confederation in the 4th century BC. The Weishu records that the Kumo Xi and Khitans spoke the same language. The Book of Wei records : The Book of Sui records: The New Book of Tang records:
By the early Tang period, the Xi had become subordinate to the Khitans. After the Khitans' Li-Sun Rebellion and revolt of Ketuyu, the Xi regained their position of dominance. The Xi then entered a golden age, lasting from approximately 755 to 847. During this period the Xi were friendly with An Lushan, and supported An in his An Shi Rebellion, plundering Han territories frequently within this period. This aggressive policy seems to have consumed Xi forces, especially weakening their demographic vitality, allowing the less aggressive Khitans to dominate them. Xi raids into China provoked successive heavy responses from the Tang, resulting in battles in the 760s and in 795 that were disastrous for the Xi. After 795, the Xi became a tributary people to the Tang. The Uyghur Empire collapsed in the 840's. When the Tang dynasty simultaneously displayed signs of division, the Xi rose in rebellion in 847, and were subsequently and disastrously defeated by Zhang Zhongwu, the frontier commander of Lulong. The Xi were never able to recover from their defeat in 847. In the late ninth century AD the Khitans rose to eventually absorb the remnants of Xi people, and established the Liao Dynasty in 907.
Cultural heritage
It is believed that the Xiqin, a bowed, stringed instrument that is the ancestor of the Chinese Erhu, the Mongolian Khuuchir and Morin khuur, was derived from a Xi instrument.