After the founding of the Tang Dynasty, the national strength was greatly reduced and the population declined. Two years after Li Shimin ascended the throne, the population of the Tang Dynasty was only 2.9 million. Therefore, Li Shimin often used the demise of the Sui Dynasty to declare himself, to be benevolent, to be clean, to pay less taxes, to avoid corvee, to unite the forces of ethnic minorities, and to formulate a series of preferential policies that benefit the country and the people. Committed to the revival of population and the prosperity of national strength, the construction of the Great Wall is not conducive to the development of national strength.
With the efforts of Li Shimin, the Tang Dynasty gradually became strong and established the rule of Zhenguan. At that time, the Tang Dynasty was recognized by the world as one of the most powerful times in China. In the Tang Dynasty, it also brought China culture to the top of the world. Countries competed to come to the Tang Dynasty to learn about oriental civilization, and once thousands of dynasties came to congratulate them. It was not unusual for Jian Zhen to travel eastward and Xuanzang to learn from the scriptures at that time in the Tang Dynasty. Businessmen from all over the world can be seen everywhere in Chang 'an market. This shows that Chang 'an at that time was the largest city in the world, so that Bai Juyi in his later years still lamented that he had struggled all his life and had not bought a house of his own in Chang 'an. Building the Great Wall is not conducive to exchanges between countries.
Li Shimin not only initiated the rule of Zhenguan, but also pushed the comprehensive national strength of the Tang Dynasty to a climax. At that time, the Tang Dynasty was the magistrate of the world, and whoever refused to accept it would be beaten, pacifying Turks, pacifying Tuyuhun, pacifying Gaochang, personally levying Goguryeo and destroying Xue Yantuo. The neighboring ethnic minorities called Li Shimin Tiankhan, and the force value was one of the best in the world at that time, so it was unnecessary to build the Great Wall.
The enlightened monarch and open folk customs in the Tang Dynasty made the Tang Dynasty a world power, and it also made the Tang Dynasty not angry and arrogant, and there was no need to build the Great Wall or worry about someone sending troops to attack it. Therefore, there is no need for Li Shimin to waste manpower and financial resources to build the Great Wall.