It was under the Emperor Wu-ti (140-87 B.C.) of the Han dynasty that the Chinese first penetrated into the Tarim basin. They had heard that the Hsiung-nu, of whose growing power they were afraid, had driven the Yüeh-chih westwards and they therefore despatched an envoy named Chang Ch'ien in the hope of inducing the Yüeh-chih to co-operate with them against the common enemy. Chang Ch'ien made two adventurous expeditions, and visited the Yüeh-chih in their new home somewhere on the Oxus. His mission failed to attain its immediate political object but indirectly had important results, for it revealed to China that the nations on the Oxus were in touch with India on one hand and with the more mysterious west on the other. Henceforth it was her aim to keep open the trade route leading westwards from the extremity of the modern Kansu province to Kashgar, Khotan and the countries with which those cities communicated. Far from wishing to isolate herself or exclude foreigners, her chief desire was to keep the road to the west open, and although there were times when the flood of Buddhism which swept along this road alarmed the more conservative classes, yet for many centuries everything that came in the way of merchandize, art, literature, and religion was eagerly received. The chief hindrance to this intercourse was the hostility of the wild tribes who pillaged caravans and blocked the route, and throughout the whole stretch of recorded history the Chinese used the same method to weaken them and keep the door open, namely to create or utilize a quarrel between two tribes. The Empire allied itself with one in order to crush the second and that being done, proceeded to deal with its former ally.