In the two decades between 160 and 140 B.C. there had been further trouble with the Hsiung-nu, though there was no large-scale fighting. There was a fundamental change of policy under the next emperor, Wu (or Wu Ti, 141-86 B.C.). The Chinese entered for the first time upon an active policy against the Hsiung-nu. There seem to have been several reasons for this policy, and several objectives. The raids of the Hsiung-nu from the Ordos region and from northern Shansi had shown themselves to be a direct menace to the capital and to its extremely important hinterland. Northern Shansi is mountainous, with deep ravines. A considerable army on horseback could penetrate some distance to the south before attracting attention. Northern Shensi and the Ordos region are steppe country, in which there were very few Chinese settlements and through which an army of horsemen could advance very quickly. It was therefore determined to push back the Hsiung-nu far enough to remove this threat. It was also of importance to break the power of the Hsiung-nu in the province of Kansu, and to separate them as far as possible from the Tibetans living in that region, to prevent any union between those two dangerous adversaries. A third point of importance was the safeguarding of caravan routes. The state, and especially the capital, had grown rich through Wen Ti's policy. Goods streamed into the capital from all quarters. Commerce with central Asia had particularly increased, bringing the products of the Middle East to China. The caravan routes passed through western Shensi and Kansu to eastern Turkestan, but at that time the Hsiung-nu dominated the approaches to Turkestan and were in a position to divert the trade to themselves or cut it off. The commerce brought profit not only to the caravan traders, most of whom were probably foreigners, but to the officials in the provinces and prefectures through which the routes passed. Thus the officials in western China were interested in the trade routes being brought under direct control, so that the caravans could arrive regularly and be immune from robbery. Finally, the Chinese government may well have regarded it as little to its honour to be still paying dues to the Hsiung-nu and sending princesses to their rulers, now that China was incomparably wealthier and stronger than at the time when that policy of appeasement had begun.

[Illustration: Map 3. China in the struggle with, the Huns or Hsiung Nu _(roughly 128-100 B.C.)_]

The first active step taken was to try, in 133 B.C., to capture the head of the Hsiung-nu state, who was called a _shan-yue_; but the _shan-yue_ saw through the plan and escaped. There followed a period of continuous fighting until 119 B.C. The Chinese made countless attacks, without lasting success. But the Hsiung-nu were weakened, one sign of this being that there were dissensions after the death of the _shan-yue_ Chuen-ch'en, and in 127 B.C. his son went over to the Chinese. Finally the Chinese altered their tactics, advancing in 119 B.C. with a strong army of cavalry, which suffered enormous losses but inflicted serious loss on the Hsiung-nu. After that the Hsiung-nu withdrew farther to the north, and the Chinese settled peasants in the important region of Kansu.

Meanwhile, in 125 B.C., the famous Chang Ch'ien had returned. He had been sent in 138 to conclude an alliance with the Yueeh-chih against the Hsiung-nu. The Yueeh-chih had formerly been neighbours of the Hsiung-nu as far as the Ala Shan region, but owing to defeat by the Hsiung-nu their remnants had migrated to western Turkestan. Chang Ch'ien had followed them. Politically he had had no success, but he brought back accurate information about the countries in the far west, concerning which nothing had been known beyond the vague reports of merchants. Now it was learnt whence the foreign goods came and whither the Chinese goods went. Chang Ch'ien's reports (which are one of the principal sources for the history of central Asia at that remote time) strengthened the desire to enter into direct and assured commercial relations with those distant countries. The government evidently thought of getting this commerce into its own hands. The way to do this was to impose "tribute" on the countries concerned. The idea was that the missions bringing the annual "tribute" would be a sort of state bartering commissions. The state laid under tribute must supply specified goods at its own cost, and received in return Chinese produce, the value of which was to be roughly equal to the "tribute". Thus Chang Ch'ien's reports had the result that, after the first successes against the Hsiung-nu, there was increased interest in a central Asian policy. The greatest military success were the campaigns of General Li Kuang-li to Ferghana in 104 and 102 B.C. The result of the campaigns was to bring under tribute all the small states in the Tarim basin and some of the states of western Turkestan. From now on not only foreign consumer goods came freely into China, but with them a great number of other things, notably plants such as grape, peach, pomegranate.

In 108 B.C. the western part of Korea was also conquered. Korea was already an important transit region for the trade with Japan. Thus this trade also came under the direct influence of the Chinese government. Although this conquest represented a peril to the eastern flank of the Hsiung-nu, it did not by any means mean that they were conquered. The Hsiung-nu while weakened evaded the Chinese pressure, but in 104 B.C. and again in 91 they inflicted defeats on the Chinese. The Hsiung-nu were indirectly threatened by Chinese foreign policy, for the Chinese concluded an alliance with old enemies of the Hsiung-nu, the Wu-sun, in the north of the Tarim basin. This made the Tarim basin secure for the Chinese, and threatened the Hsiung-nu with a new danger in their rear. Finally the Chinese did all they could through intrigue, espionage, and sabotage to promote disunity and disorder within the Hsiung-nu, though it cannot be seen from the Chinese accounts how far the Chinese were responsible for the actual conflicts and the continual changes of _shan-yue_. Hostilities against the Hsiung-nu continued incessantly, after the death of Wu Ti, under his successor, so that the Hsiung-nu were further weakened. In consequence of this it was possible to rouse against them other tribes who until then had been dependent on them--the Ting-ling in the north and the Wu-huan in the east. The internal difficulties of the Hsiung-nu increased further.

Wu Ti's active policy had not been directed only against the Hsiung-nu. After heavy fighting he brought southern China, with the region round Canton, and the south-eastern coast, firmly under Chinese dominion--in this case again on account of trade interests. No doubt there were already considerable colonies of foreign merchants in Canton and other coastal towns, trading in Indian and Middle East goods. The traders seem often to have been Sogdians. The southern wars gave Wu Ti the control of the revenues from this commerce. He tried several times to advance through Yuennan in order to secure a better land route to India, but these attempts failed. Nevertheless, Chinese influence became stronger in the south-west.

In spite of his long rule, Wu Ti did not leave an adult heir, as the crown prince was executed, with many other persons, shortly before Wu Ti's death. The crown prince had been implicated in an alleged attempt by a large group of people to remove the emperor by various sorts of magic. It is difficult to determine today what lay behind this affair; probably it was a struggle between two cliques of the gentry. Thus a regency council had to be set up for the young heir to the throne; it included a member of a Hsiung-nu tribe. The actual government was in the hands of a general and his clique until the death of the heir to the throne, and at the beginning of his successor's reign.

At this time came the end of the Hsiung-nu empire--a foreign event of the utmost importance. As a result of the continual disastrous wars against the Chinese, in which not only many men but, especially, large quantities of cattle fell into Chinese hands, the livelihood of the Hsiung-nu was seriously threatened; their troubles were increased by plagues and by unusually severe winters. To these troubles were added political difficulties, including unsettled questions in regard to the succession to the throne. The result of all this was that the Hsiung-nu could no longer offer effective military resistance to the Chinese. There were a number of _shan-yue_ ruling contemporaneously as rivals, and one of them had to yield to the Chinese in 58 B.C.; in 51 he came as a vassal to the Chinese court. The collapse of the Hsiung-nu empire was complete. After 58 B.C. the Chinese were freed from all danger from that quarter and were able, for a time, to impose their authority in Central Asia.