 Complete the following phrase:  In 139 BCE the Emperor Wu of Han dispatched the former palace attendant Zhang Qian to form an alliance with the Yuezhi people in order to combat the Xiongnu. He was captured by the Xiongnu, and held prisoner for a decade. After his return, his knowledge of the lands of the west was the main information the Han had of this region. Between 133 BCE and 89 CE, the Han and the Xiongnu fought a series of battles, known as the 