on the verge of mutiny, the Emperor had no choice but to agree, ordering the strangling of Lady Yang. The incident made Xuanzong fear for his own safety, so he fled to Chengdu at once. However, people stopped his horse, not wanting him to go away. So he made the crown prince, Li Heng, stay to hold the fort. Instead, Li Heng fled in the other direction to Lingzhou (today called Lingwu, in Ningxia province). Later, on 12 August, after reaching Sichuan, Xuanzong abdicated (becoming Taishang Huang), in favor of the crown prince, who had already been proclaimed emperor. Fall of Chang'an In July 756 An Lushan and his rebel forces captured Chang'an, an event that had a devastating effect upon this thriving metropolis. Before the revolt, estimates put the population within the city walls at from 800,000–1,000,000. Including small cities in the vicinity forming the metropolitan area, the census in 742 recorded 362,921 families with 1,960,188 persons. Much of the population fled at the approach of the rebels. Then the city was captured and looted by the rebel forces and the remaining population put in jeopardy. A new emperor The third son of Xuanzong, Li Heng, was proclaimed Emperor Suzong at Lingzhou (modern-day Lingwu), although another group of local officials and Confucian literati tried to promote a different prince, Li Lin, the Prince of Yong, at Jinling (modern-day Nanjing). One of Suzong's first acts as emperor was to appoint the generals Guo Ziyi and Li Guangbi to deal with the rebellion. The generals, after much discussion, decided to borrow troops from an offshoot of the Turkish Tujue tribe, the Huihe, or Huige, also known as the Uyghur Khaganate (ancestors of the modern-day Uyghurs, but then located in Mongolia), who were ruled by Bayanchur Khan until his death in the summer of 759. Three thousand Arab mercenaries were sent by the Abbasid caliph al-Mansur to join the Tang in 756 against An Lushan. With Uyghur assistance, the Tang Imperial forces recaptured both Chang'an and Luoyang in late 757. However, they failed to capture or subdue the rebel troops, who fled to the rebel heartland in the northeast. Uyghur Khaganate diplomats clashed against Arab Abbasid diplomats over who would enter the diplomatic hall in Chang'an first in 758. Role of Nestorian Christians and Esoteric Buddhists against An Lushan The Nestorian Church of the East Christians like the Bactrian Priest Yisi of