When the emperor Su Tsung died, in 762, Tengri, the khan of the Uighurs, decided to make himself ruler over China.  The events of the preceding years had shown him that China alone was entirely defenceless.  Part of the court clique supported him, and only by the intervention of P’u-ku Huai-en, who was related to Tengri by marriage, was his plan frustrated.  Naturally there were countless intrigues against P’u-ku Huai-en.  He entered into alliance with the Tibetan T’u-fan, and in this way the union of Turks and Tibetans, always feared by the Chinese, had come into existence.  In 763 the Tibetans captured and burned down the western capital, while P’u-ku Huai-en with the Uighurs advanced from the north.  Undoubtedly this campaign would have been successful, giving an entirely different turn to China’s destiny, if P’u-ku Huai-en had not died in 765 and the Chinese under Kuo Tz[)u]-i had not succeeded in breaking up the alliance.  The Uighurs now came over into an alliance with the Chinese, and the two allies fell upon the Tibetans and robbed them of their booty.  China was saved once more.

Friendship with the Uighurs had to be paid for this time even more dearly.  They crowded into the capital and compelled the Chinese to buy horses, in payment for which they demanded enormous quantities of silkstuffs.  They behaved in the capital like lords, and expected to be maintained at the expense of the government.  The system of military governors was adhered to in spite of the country’s experience of them, while the difficult situation throughout the empire, and especially along the western and northern frontiers, facing the Tibetans and the more and more powerful Kitan, made it necessary to keep considerable numbers of soldiers permanently with the colours.  This made the military governors stronger and stronger; ultimately they no longer remitted any taxes to the central government, but spent them mainly on their armies.  Thus from 750 onward the empire consisted of an impotent central government and powerful military governors, who handed on their positions to their sons as a further proof of their independence.  When in 781 the government proposed to interfere with the inheriting of the posts, there was a great new rising, which in 783 again extended as far as the capital; in 784 the T’ang government at last succeeded in overcoming it.  A compromise was arrived at between the government and the governors, but it in no way improved the situation.  Life became more and more difficult for the central government.  In 780, the “equal land” system was finally officially given up and with it a tax system which was based upon the idea that every citizen had the same amount of land and, therefore, paid the same amount of taxes.  The new system tried to equalize the tax burden and the corvee obligation, but not the land.  This change may indicate a step towards greater freedom for private enterprise.  Yet it did not benefit the government, as most of the tax income was retained by the governors and was used for their armies and their own court.