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._)]