The Han Dynasty of China (202 BCE – 220 CE) was regularly harassed by the nomadic tribes of the Xiongnu on their northern and western borders. In 138 BCE, Emperor Wu sent his emissary Zhang Qian to the west to negotiate with the Yuezhi people for help in defeating the Xiongnu. Zhang Qian’s expedition led him into contact with many different cultures and civilizations in central Asia and, among them, those whom he designated the `Dayuan’, the `Great Ionians’, who were the Greco-Bactrians descended from Alexander the Great’s army. The Dayuan had mighty horses, Zhang Qian reported back to Wu, and these could be employed effectively against the marauding Xiongnu. The consequences of Zhang Qian’s journey was not only further contact between China and the west but an organized and efficient horse breeding program throughout the land in order to equip a cavalry. The horse had long been known in China and had been used in warfare for cavalry and chariots as early as the Shang Dynasty (1600 – 1046 BCE) but the Chinese admired the western horse for its size and speed. With the western horse of the Dayuan, the Han Dynasty defeated the Xiongnu. This success inspired the Emperor Wu to speculate on what else might be gained through trade with the west and the Silk Road was opened in 130 BCE.