In the fifth year of Tianbao's reign (746), Emperor Xuanzong of the Tang Dynasty, Wang Zhongsi, the governor of Hexi Province, died violently and was demoted to his post. The northwest frontier defense fell into a power vacuum. Tubo Zanpu Chide Zuzan took the opportunity to unite with the Turks, Tuyuhun and other tribes to form an army of 200,000 outside Yumen Pass. At this time, the power struggle between Li Linfu and Yang Guozhong in the imperial court became fierce. The supply of food and grass to the border army was cut off, and the Hexi Corridor was in danger.
