The Tang Dynasty reached the summit of its prosperity in the years of Kaiyuan and Tianbao. Being in a peaceful era, the ruling class went decaying and a crisis was hidden in the peace. In the 14th year of Tianbao (755), An Lushan, who served as three military commissioners including that of Fanyang, rebelled in Youzhou (present-day Beijing) and commanded his troops southward. The long period of peace in the dynasty resulted in the weakening of its soldiers in fighting, so the government troops kept being defeated. With the fall of both capitals of Luoyang and Chang’an, Emperor Xuan fled to Chengdu. In order to defend against the rebels, the dynasty mobilized a strong force from Hexi, Longyou, Anxi, and Beiting to suppress the revolt in the Central Plains, leaving the northwest region empty of defensive power. The Tubo Kingdom on the Qinghai-Tibet Plateau took advantage of the situation and attacked the Tang territory up from Qinghai.