32 THE CONTACT OF CIVILIZATIONS CH. II
constantly subject to being disturbed by restless Turkish tribes, such as are still hovering now between the Altai and Tien-shan. More serious still was the danger presented by aggression on the part of the Tibetans who were then rapidly growing into a new military power.
Towards the middle of the eighth century there was added to the pressure from the Tibetans in the south fresh danger in the west from the steady advance of Arab conquest in the Oxus basin. The Tibetans were endeavouring to join hands with the Arabs as common foes of China's Central-Asian supremacy. By pushing down the Indus valley and thence across the Hindukush territories which correspond to the present Gilgit and Yasin they actually reached the uppermost Oxus valley. This junction threatened the Chinese position
in the Tarim basin with being outflanked simultaneously l
both on the east and the west. The endeavour to avert this serious strategic risk led to a remarkable military expedition conducted A.D. 747 by the Chinese general Kao Hsien-chih right across the `Roof of the World', the Pamirs, and the ice-crowned Hindukush pass of the Darkot. In Chapters iii, xx I shall give some details about this memorable Chinese enterprise. It stands out as a striking proof of Chinese
capacity for overcoming by organization formidable geo- a
graphical obstacles.
The prestige accruing to the Chinese arms from Kao Hsien-chih's expedition was deservedly great. But it did not save them from being signally worsted two years later. In a battle near Tashkend Kao Hsien-chih was completely defeated by the Arabs and the revolted Turkish tribes, their allies. About A.D. 75o the Tibetans from the south secured mastery over Tun-huang and the adjoining tracts at the
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