When he found himself master at home, he turned his attention to the task of protecting his frontier from aggressors. Of these, the Hiung-nu, a Tartar tribe whose habitat was Eastern Mongolia, were the most troublesome. He carried the war into the enemy’s camp by despatching an army across the great Gobi Desert, with orders to establish a strong place at Hami.[33] In B.C. 250 he commenced a work which had a more lasting effect in repressing their invasion. This was the Great Wall of China, which starts from the Shan-hi Pass and ends at the Chin-Yü barriers, a distance of not less than 1500 miles. The Hiung-nu, like their kinsmen the Mongols of Chingiz and of Tīmūr, fought on horseback, and their plan of campaign was simply a succession of raids followed by speedy retreats. This stupendous barrier intimidated them, and turned westwards the tide of their migration. Thus the Great Wall, which it is the fashion to decry as a monument of misplaced labour, was a most important factor in the history of Central Asia. At this epoch the Sakas were settled in Hexapolis, to the east of the Pamirs; while the Usuns dwelt on the southern side of Lake Lob, separated from the Sakas by the Uīghūrs. About B.C. 300 the empire of the _Yué-Chi_,[34] who were a branch of the Tung-nu, or Eastern Tartars, extended most probably from the Muztagh Mountains on the north to the Kuen-lun Mountains on the south, and from the Upper Hoang-ho in Shan-si on the east to Koché and Khotan on the west.[35] About B.C. 200 a war broke out between the Tung-nu and the Hiung-nu (the Western Tartars or Huns), their neighbours. Mothé, the chief of these latter, falling on the Eastern Tartars unawares, utterly defeated them and drove the Yué-Chi from their kingdom. The latter fled to the banks of the Ili River, while Mothé pushed his conquests as far as the Volga on the west and the border provinces of China eastwards. The Emperor Kao-tsu (B.C. 202–194), founder of the famous Han dynasty, who had achieved the subjugation of the whole of China, was alarmed at the progress of Mothé, and marched against him. His troops were, however, surrounded by Mothé’s colossal hordes in the north of the province of Shan-si, and only escaped destruction by the employment of a ruse.[36] On the departure of the Chinese army Mothé set out for Tartary. For upwards of fifty years the power of Hiung-nu sustained no check. They continued to press down on the Yué-Chi, who, after suffering a further crushing defeat, broke into separate hordes. The lesser division, or “Little Yué-Chi,” passed into Tibet. The “Great Yué-Chi’s” first movement was westwards to the banks of the Ili, but finding the Usun too strong for them, they wandered in a southerly direction, and finally descended upon Kāshghar, Yarkand, and Khotan, whence they displaced the Sakas (B.C. 163). The latter, on their expulsion from Soghdiana, invaded Bactria, and from this period until