Why did Emperor Wu of the Han Dynasty recruit envoys to the Western Regions?

Since the Qin and Han Dynasties unified the ministries, the Huns have become increasingly powerful, controlling vast areas in the north, northeast and northwest of China. At the beginning of Han Dynasty, Xiongnu attacked from time to time. Constrained by national strength, the Han Dynasty basically adopted the defense policy of "hiding relatives", supplemented by immigration. After Emperor Wu of the Han Dynasty ascended the throne, centralization was greatly strengthened, and the country's economic strength was unprecedented. The time is ripe to fight back against the Huns and eliminate their threat to the northern part of the Han Dynasty. In order to cut off the Xiongnu's right arm, Emperor Wu of the Han Dynasty openly recruited envoys to the Western Regions and sought alliances.