What was the purpose of Liang Wudi's initial dispatch of Zhang Qian to the Western Regions?

At that time, the Han Dynasty was preparing to wage war against the Huns. By chance, Emperor Wu of the Han Dynasty learned from the mouth of a Hun prisoner that there was a Yue kingdom in the Western Regions, whose king was killed by Hun Khan and his head was made into a wine vessel. The Vietnamese couldn't stand the slavery of the Huns and moved to the Ili Valley at the northern foot of Tianshan Mountain. It was attacked by Wusunguo and then moved southwest to Guishui (now Amu Darya) basin. Wang Yue wanted to avenge his father, but no one helped him. Liang Wudi, after understanding these situations, wanted to unite the Yues to "break the Hungarian right arm". So I decided to send a special envoy to Dayue. Zhang Heng was recruited as a Langguan, shouldering the task of sending Yue's family.