Emperor Xuanzong of Tang (Li Longji)

The longest-reigning of the Tang emperors, Xuanzong (685-762, r. 712-756) presided over the dynasty's greatest prosperity and then over the rebellion that permanently weakened it. Born Li Longji, a son of Emperor Ruizong and grandson of Wu Zetian, he first came to prominence in 710, when he and his aunt the Princess Taiping led the coup that destroyed Empress Wei's faction after the death of Emperor Zhongzong. Ruizong was restored, and in 712 he abdicated to his son; the following year Xuanzong eliminated the Princess Taiping's party and assumed full control.

The first half of the reign, the Kaiyuan era (713-741), is conventionally treated as the high point of the Tang. Guided by chancellors of unusual ability, notably Yao Chong and Song Jing, the government reformed coinage and registration, pruned the privileges accumulated by palace favorites, and restored fiscal order. Chang'an, with a population near a million, stood at the center of trade routes reaching Central Asia, and the court of this period patronized the poets Li Bai and Du Fu and an unprecedented flowering of music and painting.

The later reign, the Tianbao era, saw a gradual withdrawal of the aging emperor from administration. Power was concentrated first in the chancellor Li Linfu and then in Yang Guozhong, cousin of the emperor's favored consort Yang Guifei, who had earlier been married to one of Xuanzong's own sons. Frontier defense was entrusted to professional armies under non-Chinese generals, among them An Lushan, who commanded three northeastern garrisons and stood high in the emperor's favor.

In 755 An Lushan rebelled, taking Luoyang and, in 756, forcing the court to flee Chang'an toward Sichuan. At the Mawei post station the escorting soldiers killed Yang Guozhong and compelled the emperor to order the death of Yang Guifei, an episode endlessly elaborated in later literature, most famously in Bai Juyi's "Song of Everlasting Sorrow." The crown prince broke away and was proclaimed Emperor Suzong, and Xuanzong accepted the status of retired emperor. He returned to a recaptured Chang'an in 757 and lived out his final years in seclusion, dying in 762 while the rebellion still burned. The dynasty survived another century and a half, but the centralized order of the Kaiyuan years was never rebuilt.