First of all, Taizong lost his virtue at home, which also harmed the country to a certain extent. In the autocratic era, China's political feature is "family and the world", and the foundation of the monarch's rule of the country is naturally the rule of the country, so the ethics of monarch, minister and son is the foundation of governing the country. However, Emperor Taizong attacked and killed his younger brother, forced his father to meditate, robbed his uncle Li Yuan's concubine and robbed his younger brother Yuanji's wife. It can be seen that Emperor Taizong did not have the virtue of governing the world with benevolence and filial piety in feudal times. Since then, examples of incest, waste storage, destruction of imperial clan and chaos of home and country have emerged one after another. Ru Song said that "Tang Sangang was dishonest and had no husband and wife, which originated from Taizong", which is a definite theory. From this point of view, it is not necessary for Emperor Taizong to create a peaceful and prosperous time.
Secondly, the repeated partiality of Emperor Taizong made Zhenguan politics lose its statutes. When Hou broke Gaochang, he led his troops to plunder treasures, which led to the evil result that "several counties were bare and time and tide wait for no man" and was put into prison according to law. Because Cen Wenben interceded, Taizong let people go. Later, Emperor Taizong ordered the crown prince to take the treasure from the treasury, which was not restricted by the company, so the prince took it too much and his son wrote a letter to remonstrate. At the beginning of the dynasty, the prince sent someone to beat him and almost killed him! Taizong did not pursue it. In the same year, Hong Ren, a greedy political party, committed a crime, damn it. Taizong personally put on a show to save his life. Aside from the glorious quotations of Emperor Taizong on the legal system in the history books, there are many such examples to compare and study the arbitrariness in the process of law enforcement during the Zhenguan period.
Third, when Emperor Taizong was in power, he always dreamed of restoring the feudal vassal system, which has been repeatedly proved by history to be the most likely to lead to civil strife and national subjugation. In this regard, Emperor Taizong himself is not only unwise, but may even bring disaster to the country.
Fourth, Emperor Taizong's blunder in the frontier and his arrogance harmed the people, which led to serious consequences of external disturbances and civil strife. In the first year of Zhenguan, the states declared that Feng Ang in Lingnan had rebelled, and Emperor Taizong ordered general Jiang and Ling to send soldiers from dozens of states, which almost led to an unjust war that might bring disaster to South China. After the sixteen-year reign of Zhenguan, Emperor Taizong refused to accept the advice of Gaochang's children as princes. Instead, he sent troops to defend the land, wasting people and money, leading to the Turkish invasion. In the seventeenth year of Zhenguan, Xue Yantuo, who was really close, was first allowed to get married and then cheated, but with malice, he buried the future trouble of diplomatic dishonesty. A few years later, after the defeat of Korea, Xue Yantuo led the troops to the border, and more than a dozen States in the Tang Dynasty were restless. In the eighteenth year of Zhenguan, Emperor Taizong did not listen to advice and attacked Korea on a large scale many times. Finally, the soldiers were exhausted and came back in a mess. Emperor Taizong repented but never repented, and then launched a larger-scale war. In order to create a large number of naval warships to attack North Korea, it not only served the people of twelve states in the south of the Yangtze River, but also affected the ecological environment and social stability in southwest China. Finally, in the case of "Shu people's bitter shipbuilding war", the governors of prefectures and counties were forced to be strict and urgent, and the people could not afford to sell farmland houses and children, and the food prices were expensive, which caused the people of Ya, Qiong and Mei to rebel. The court had to spend a lot of manpower and material resources to quell the civil strife, and Emperor Taizong himself died of sudden illness in the voice of the people's resistance. Because Emperor Taizong was overjoyed, he threatened the people and disturbed the outside world, almost repeating the mistakes of the Sui Dynasty.