Why didn't the Western Regions be discovered until Emperor Wudi of the Han Dynasty fought fierce slaves to unite with the Western Regions?

It was not the discovery of the Western Regions, but the fact that before Emperor Wu, the Han people knew little about the countries in the Western Regions and had no contacts.

From the Xiongnu population, Emperor Wu of the Han Dynasty learned that there once lived a nomadic people named Dayue in Dunhuang and Qilian, which was called "Yu's" in China ancient books. During the Qin and Han Dynasties, the Yue people became powerful, captured the land of Wusun, a neighboring country, and clashed with Xiongnu. At the beginning of the Han Dynasty, the country was defeated by Xiongnu's modu chanyu for many times, and the national situation declined day by day. To the old Khan, was completely conquered by the Huns. Lao Shang Khan killed King Yue and cut off his head to make wine vessels. After this national disaster, the Yue people were forced to move westward. In today's Yili area in northwest Xinjiang, the original "Serbs" were driven away and the country was re-established. But they don't forget their homeland, they are always ready to take revenge on the Huns, and they really want someone to help them. According to this situation, Emperor Wudi of the Han Dynasty decided to unite with Yue's family and attack the Huns with * * *, and ordered Zhang Qian to send an envoy to Zhang Qian in the Western Regions to Dawan. After that, he explained to King Dawan his mission of sending Yue's family and all kinds of experiences along the way, hoping that Dawan could send someone to see him off, and said that if he could return to the Han Dynasty in the future, he would definitely play the Ming Emperor and give him a lot of property and rewards. King Dawan had already heard about the wealth of the Eastern Han Dynasty, and he wanted to communicate with the Han Dynasty, but he was hindered by the middle stalk of the Xiongnu and failed to achieve it. Han's unexpected arrival made him very happy. Zhang Qian's words, more tempted him. So he readily agreed to Zhang Qian's request, and after a warm reception, he sent a guide and an interpreter to send Zhang Qian and others to live in comfort (now in Uzbekistan and Tajikistan of the Soviet Union). King Kangju sent someone to send them to Dayue.

Unexpectedly, at this time, because the new land is very fertile and rich in products, and it is far away from Xiongnu and Wusun, the danger of foreign enemies has been greatly reduced and their attitude has changed. When Zhang Qian suggested to them, they had no intention of taking revenge on the Huns. In addition, they thought that the Han Dynasty was too far away from the Yue people, and if they jointly attacked the Huns, it would be difficult to help them in case of danger. Zhang Qian and others stayed in Yueren for more than a year, but failed to persuade Yueren to ally with the Han Dynasty and attack the Huns.

Zhang Qian's expedition was not completed as far as the scheduled mission to the western regions was concerned. Because he failed to achieve the goal of establishing an alliance with the big moon family to attack the Huns. For example, it is undoubtedly a great success in terms of its practical influence and historical role. Since the Spring and Autumn Period, Rongdi has lived in the north of Jingwei River. To the north of Qin Shihuang, the Great Wall was built to protect the Central Plains, but its western boundary was just the vast western region outside Lintao and Yumen, which was still beyond the reach of our political and cultural forces. Zhang Qian's first visit to the Western Regions enabled China's influence to reach the east and west of the Green Ridge. Since then, not only the links between Xinjiang and the mainland have been strengthened, but also the direct contacts between China and Central Asia, West Asia and even Southern Europe have been established and close.