Xuanzong’s reign (712–756) was the high point of the Tang dynasty. It was an era of wealth and prosperity that was accompanied by institutional progress and a flowering of the arts. Political life was at first dominated by the bureaucrats recruited through the examination system who had staffed the central government under Wuhou. But a gradual revival of the power of the great aristocratic clans tended to polarize politics, a polarization that was sharpened by the emperor’s employment of a series of aristocratic specialists who reformed the empire’s finances from 720 onward, often in the teeth of bureaucratic opposition.

After 720 a large-scale re-registration of the population greatly increased the number of taxpayers and restored state control over vast numbers of unregistered families. The new household and land taxes were expanded. In the 730s the canal system, which had been allowed to fall into neglect under Wuhou and her successors, was repaired and reorganized so that the administration could transport large stocks of grain from the Yangtze region to the capital and to the armies on the northern frontiers. The south was at last financially integrated with the north. By the 740s the government had accumulated enormous reserves of grain and wealth. The tax and accounting systems were simplified, and taxes and labor services were reduced.

Some important institutional changes accompanied these reforms. The land registration, reorganization of transport, and coinage reform were administered by specially appointed commissions holding extraordinary powers, including the authority to recruit their own staff. These commissions were mostly headed by censors, and they and the censorate became centers of aristocratic power. The existence of these new offices reduced the influence of the regular ministries, enabling the emperor and his aristocratic advisers to circumvent the normal channels and procedures of administration.

After 736 the political dominance of the aristocracy was firmly reestablished. An aristocratic chief minister, Li Linfu, became a virtual dictator, his powers increasing as Xuanzong in his later years withdrew from active affairs into the pleasures of palace life and the study of Daoism. In the latter part of his reign, Xuanzong, who had previously strictly circumscribed the power of the palace women to avoid a recurrence of the disasters of Wuhou’s time and who had also excluded members of the royal family from politics, faced a series of succession plots. In 745 he fell deeply under the influence of a new favorite, the imperial concubine Yang Guifei. In 751–752 one of her relatives, Yang Guozhong, thanks to her influence with the emperor, rapidly rose to rival Li Linfu for supreme power. After Li’s death in 752 Yang Guozhong dominated the court. However, he had neither Li’s great political ability nor his experience and skill in handling people.

Military reorganization

The most important new development in Xuanzong’s reign was the growth in the power of the military commanders. During Gaozong’s reign the old militia system had proved inadequate for frontier defense and had been supplemented by the institution of permanent armies and garrison forces quartered in strategic areas on the frontiers. These armies were made up of long-service veterans, many of them non-Chinese cavalry troops, settled permanently in military colonies. Although these armies were adequate for small-scale operations, for a major campaign an expeditionary army and a headquarters staff had to be specially organized and reinforcements sent in by the central government. This cumbersome system was totally unsuitable for dealing with the highly mobile nomadic horsemen on the northern frontiers.

At the beginning of Xuanzong’s reign, the Turks again threatened to become a major power, rivaling China in Central Asia and along the borders. Kapghan (Mochuo), the Turkish khan who had invaded Hebei in the aftermath of the Khitan invasion in the time of Wuhou and had attacked the Chinese northwest at the end of her reign, turned his attention northward. By 711 he controlled the steppe from the Chinese frontier to Transoxiana and appeared likely to develop a new unified Turkish empire. When he was murdered in 716, his flimsy empire collapsed. His successor, Bilge (Pijia), tried to make peace with the Chinese in 718, but Xuanzong preferred to try to destroy his power by an alliance with the southwestern Basmil Turks and with the Khitan in Manchuria. Bilge, however, crushed the Basmil and attacked Gansu in 720. Peaceful relations were established in 721–722. Bilge’s death in 734 precipitated the end of Turkish power. A struggle among the various Turkish subject tribes followed, from which the Uighurs emerged as victors. In 744 they established a powerful empire that was to remain the dominant force on China’s northern border until 840. Unlike the Turks, however, the Uighurs pursued a consistent policy of alliance with the Tang. On several occasions Uighur aid, even though offered on harsh terms, saved the dynasty from disaster.

The Tibetans were the most dangerous foe during Xuanzong’s reign, invading the northwest annually from 714 on. In 727–729 the Chinese undertook large-scale warfare against them, and in 730 a settlement was concluded. But in the 730s fighting broke out again, and the Tibetans began to turn their attention to the Tang territories in the Tarim Basin. Desultory fighting continued on the border of Gansu until the end of Xuanzong’s reign. From 752 onward the Tibetans acquired a new ally in the Nanzhao state in Yunnan, which enabled them to exert a continuous threat along the entire western frontier.

In the face of these threats, Xuanzong organized the northern and northwestern frontiers from Manchuria to Sichuan into a series of strategic commands or military provinces under military governors who were given command over all the forces in a large region. This system developed gradually and was formalized in 737 under Li Linfu. The frontier commanders controlled enormous numbers of troops: nearly 200,000 were stationed in the northwest and Central Asia and more than 100,000 in the northeast; there were well in excess of 500,000 in all. The military governors soon began to exercise some functions of civil government. In the 740s a non-Chinese general of Sogdian and Turkish origin, An Lushan, became military governor first of one and finally of all three of the northeastern commands, with 160,000 troops under his orders. An Lushan had risen to power largely through the patronage of Li Linfu. When Li died, An became a rival of Yang Guozhong. As Yang Guozhong developed more and more of a personal stranglehold over the administration at the capital, An Lushan steadily built up his military forces in the northeast. The armed confrontation that followed nearly destroyed the dynasty.

During the 750s there was a steady reversal of Tang military fortunes. In the far west the overextended imperial armies had been defeated by the Arabs in 751 on the Talas River. In the southwest a campaign against the new state of Nanzhao had led to the almost total destruction of an army of 50,000 soldiers. In the northeast the Chinese had lost their grip on the Manchuria-Korea border with the emergence of the new state of Parhae in place of Koguryŏ, and the Khitan and Xi peoples in Manchuria constantly caused border problems. The Tibetans in the northwest were kept in check only by an enormously expensive military presence. The principal military forces were designed essentially for frontier defense.

Thus, the end of Xuanzong’s reign was a time when the state was in a highly unstable condition. The central government was dangerously dependent on a small group of men operating outside the regular institutional framework, and an overwhelming preponderance of military power was in the hands of potentially rebellious commanders on the frontiers, against whom the emperor could put into the field only a token force of his own and the troops of those commanders who remained loyal.