CH. II CENTRAL ASIA UNDER TANG 31
stream of nomadic migration which was moving westwards along the Tien-shan, slowly at one time, more rapidly at another, had put the great confederation of Turkish tribes, known to the Chinese as the Western Turks, in ascendancy over the vast Central-Asian region previously dominated by the White Huns. Like their allies, the Northern Turks, they were in the east troublesome neighbours to the Chinese Empire, which by A.D. 589 had after nearly three centuries of division become once more united.
The gradual consolidation of Chinese power which continued after the accession of the great Tang dynasty by A.D. 618 was at first accompanied by a policy of rigid seclusion on the north-western marches. But this was soon to give way to a `forward policy' on a grand scale which for over a century made Chinese imperial power under the Tang dynasty expand over wider regions of Central Asia than it had ever before. The power of the Western Turks was already weakened by tribal dissensions which Chinese diplomacy skilfully fostered. First Hami and subsequently Turfan were wrested from Turkish supremacy. By A.D. 66o it was finally shattered by the Emperor Kao-tsung's forces. China thus succeeded to the Western Turks' claim over a vast dominion extending from the Altai Mountains to beyond the Hindukush.
But the fact of China now claiming succession to the wide dominions once held by the Western Turks was bound to prove in time a source of trouble and weakness. The Chinese forces stationed in what were called `the Four Garrisons' had to guard not merely the oases of the Tarim basin, but also territories to the north of the Tien-shan. These offered attractive grazing-grounds to nomads, and were hence
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