revised figure is repeated by cultural historians such as Johan Norberg. Historians such as Charles Patrick Fitzgerald argue that claims of massive depopulation are incompatible with contemporary accounts of the war, which was fought intermittently over three or four provinces. They point out that the numbers recorded on the postwar registers reflect not only population loss, but also a breakdown of accuracy of the census system as well as the removal from the census figures of various classes of untaxed persons, such as those in religious orders, foreigners and merchants. In addition, several of the northern provinces, with approximately a quarter of the empire's population, were no longer subject to the imperial revenue system. For these reasons, census numbers for the post-rebellion Tang are considered unreliable. The An Lushan Rebellion was one of several wars in northern China along with the Uprising of the Five Barbarians, Huang Chao Rebellion, the wars of the Five Dynasties and Ten Kingdoms and Jin–Song Wars which caused a mass migration of Han Chinese from northern China to southern China called 衣冠南渡(yì guān nán dù). These mass migrations led to southern China's population growth, economic, agricultural and cultural development as it stayed peaceful unlike the north. Weakening of Tang The rebellion of An Lushan and its aftermath greatly weakened the centralized bureaucracy of the Tang dynasty, especially in regards to its perimeters. Virtually autonomous provinces and ad hoc financial organizations arose, reducing the influence of the regular bureaucracy in Chang'an. The Tang dynasty's desire for political stability in this turbulent period also resulted in the pardoning of many rebels. Indeed, some were even given their own garrisons to command. Political and economic control of large swathes of the empire became intermittent or was lost, and these areas came to be controlled by fanzhen, autonomous regional authorities headed by the jiedushi (regional military governors). In Hebei, three fanzhen became virtually independent for the remainder of the Tang dynasty. Furthermore, the Tang government also lost most of its control over the Western Regions, due to troop withdrawal to central China to attempt to crush the rebellion and deal with subsequent disturbances. Continued military and economic weakness resulted in further subsequent erosions of Tang territorial control during the ensuing years, particularly in regard to the Tibetan empires. By 790 Chinese control over the Tarim Basin area was completely lost. Moreover, during the rebellion, the Tang general Gao Juren