Emperor Xianzong of Tang in the context of Liu Pi (official)

The 9th century was a period from 801 (represented by the Roman numerals DCCCI) through 900 (CM) in accordance with the Julian calendar.

The Carolingian Renaissance and the Viking raids occurred within this period. In the Middle East, the House of Wisdom was founded in Abbasid Baghdad, attracting many scholars to the city. The field of algebra was founded by the Muslim polymath al-Khwarizmi. The most famous Islamic scholar Ahmad ibn Hanbal was tortured and imprisoned by Abbasid official Ahmad ibn Abi Du'ad during the reign of Abbasid caliph al-Mu'tasim and caliph al-Wathiq. In Southeast Asia, the Mataram kingdom reached its zenith, while Burma would see the establishment of the major kingdom of Pagan. Tang China started the century with the effective rule of Emperor Xianzong and ended it with the Huang Chao Rebellion. In America, the Maya experienced widespread political collapse in the central Maya region, resulting in internecine warfare, the abandonment of cities, and a northward shift of population.

The Three Fanzhen of Hebei (Chinese: 河朔三镇; pinyin: Hé shuò sān zhèn) were three regions in what is now Hebei, China governed by powerful jiedushi in the post-An Lushan Rebellion (755-763) Tang dynasty (618–907). After the rebellion, the three regions of Chéngdé (成德), Lúlóng (盧龍) and Wèibó (魏博) were controlled by ex-rebel generals who held substantial territory and forces. Although nominally under the authority of the Tang dynasty, they were functionally independent fanzhen that continued to the end of the Tang dynasty. In the south, however, the court took a much more aggressive stand against such defense commands as Zīqīng (淄青) (mainly in Shandong), Biànsòng (汴宋) (in east Henan), and Huáixī (淮西) (in south Henan), which posed a more immediate and palpable threat to the transportation of strategic grains through the Grand Canal, on which the court depended.

During Emperor Xianzong's reign the northeast region was briefly subdued, but after his death it became independent again. By the reign of Emperor Wenzong, the central government had lost all control over this region. The situation was summed up thus by Tang dynasty's chancellor Niu Sengru: