When the T'ang dynasty (618-907) re-united the Empire, the Chinese Government with characteristic tenacity reverted to its old policy of keeping the western road open and to its old methods. The Turks were then divided into two branches, the northern and western, at war with one another. The Chinese allied themselves with the latter, defeated the northern Turks and occupied Turfan (640). Then in a series of campaigns, in which they were supported by the Uigurs, they conquered their former allies the western Turks and proceeded to organize the Tarim basin under the name of the Four Garrisons[485]. This was the most glorious period of China's foreign policy and at no other time had she so great a position as a western power. The list of her possessions included Bokhara in the west and starting from Semirechinsk and Tashkent in the north extended southwards so as to embrace Afghanistan with the frontier districts of India and Persia[486]. It is true that the Imperial authority in many of these regions was merely nominal: when the Chinese conquered a tribe which claimed sovereignty over them they claimed sovereignty themselves. But for the history of civilization, for the migration of art and ideas, even this nominal claim is important, for China was undoubtedly in touch with India, Bokhara and Persia. But no sooner did these great vistas open, than new enemies appeared to bar the road. The Tibetans descended into the Tarim basin and after defeating the Chinese in 670 held the Four Garrisons till 692, when the fortunes of war were reversed. But the field was not left clear for China: the power of the northern Turks revived, and Mohammedanism, then a new force but destined to ultimate triumph in politics and religion alike, appeared in the west. The conquests of the Mohammedan general Qutayba (705-715) extended to Ferghana and he attacked Kashgar. In the long reign of Hsüan Tsung China waged a double warfare against the Arabs and Tibetans. For about thirty years (719-751) the struggle was successful. Even Tabaristan is said to have acknowledged China's suzerainty. Her troops crossed the Hindu Kush and reached Gilgit. But in 751 they sustained a crushing defeat near Tashkent. The disaster was aggravated by the internal troubles of the Empire and it was long before Chinese authority recovered from the blow[487]. The Tibetans reaped the advantage. Except in Turfan, they were the dominant power of the Tarim basin for a century, they took tribute from China and when it was refused sacked the capital, Chang-an (763). It would appear however that for a time Chinese garrisons held out in Central Asia and Chinese officials exercised some authority, though they obtained no support from the Empire[488]. But although even late in the tenth century Khotan sent embassies to the Imperial Court, China gradually ceased to be a Central Asian power. She made a treaty with the Tibetans (783) and an alliance with the Uigurs, who