west provided even greater incentive to counter the Xiongnu hold on westward routes out of the Han Empire, and the Han prepared to mount a large scale attack using the Northern Silk Road to move men and material. While the Han dynasty was making preparations for a military confrontation since the reign of Emperor Wen, the break did not come until 133 BC, following an abortive trap to ambush the chanyu at Mayi. By that point the empire was consolidated politically, militarily and economically, and was led by an adventurous pro-war faction at court. In that year, Emperor Wu reversed the decision he had made the year before to renew the peace treaty. Full-scale war broke out in autumn 129 BC, when 40,000 Han cavalry made a surprise attack on the Xiongnu at the border markets. In 127 BC, the Han general Wei Qing retook the Ordos. In 121 BC, the Xiongnu suffered another setback when Huo Qubing led a force of light cavalry westward out of Longxi and within six days fought his way through five Xiongnu kingdoms. The Xiongnu Hunye king was forced to surrender with 40,000 men. In 119 BC both Huo and Wei, each leading 50,000 cavalrymen and 100,000 footsoldiers (in order to keep up with the mobility of the Xiongnu, many of the non-cavalry Han soldiers were mobile infantrymen who traveled on horseback but fought on foot), and advancing along different routes, forced the chanyu and his Xiongnu court to flee north of the Gobi Desert. Major logistical difficulties limited the duration and long-term continuation of these campaigns. According to the analysis of Yan You (嚴尤), the difficulties were twofold. Firstly there was the problem of supplying food across long distances. Secondly, the weather in the northern Xiongnu lands was difficult for Han soldiers, who could never carry enough fuel. According to official reports, the Xiongnu lost 80,000 to 90,000 men, and out of the 140,000 horses the Han forces had brought into the desert, fewer than 30,000 returned to the Han Empire. In 104 and 102 BC, the Han fought and won the War of the Heavenly Horses against the Kingdom of Dayuan. As a result, the Han gained many Ferghana horses which further aided them in their battle against the Xiongnu. As a result of these battles, the Han Empire controlled the strategic region from the Ordos and Gansu corridor to Lop Nor.