Li Shimin, who, at the age of sixteen, already showed his outstanding military ability, persuaded his father to borrow military power from a formal enemy country of the Sui Dynasty -- the Hun.

Before long, Li Yuan left Jin Yang and headed for Chang'an together with 30,000 soldiers. After the army taking Chang'an in a flash, Li Yuan made Yang You, son of Sui Yangdi, nothing but a temporary puppet emperor. When Sui Yangdi was assassinated by an aide in 618, Li Yuan deposed the puppet prince and declared himself emperor, and changed the name of the empire to the Tang Dynasty (618-907). The following years he was assisted by Li Shimin who continued to command the rebel armies and conquered all other rebellions, consolidating the kingdom.

On taking the title and name Emperor Gauzou, Li Yuan decreed that his eldest son, Li Jiancheng, should be the Crown Prince. Li Shimin, the second son, was granted the title Prince of Qin. A third son had died long before, so the fourth son, Li Yuanji was granted Prince of Qi.

Of these three sons, Li Shimin was the most ambitious and intelligent. It was he who had made the greatest contribution to the campaign to secure the empire for his father. This fact troubled the Crown Prince for he saw Li Shimin as a threat to his own position. Conspiring with his younger brother, Prince Qi, he tried to get rid of Li Shimin.

Unhappy with his father's incompetence and tired of the constant threats against him by his brothers, Li Shimin launched a palace coup at the Xuanwu Gate in 626. Both the Crown Prince and Prince Qi were killed. This was recorded as The Xuanwu Gate Coup. As a result, Li Yuan was then compelled to abdicate in favor of Li Shimin, and the latter ascended the throne as Emperor Taizong, whose reign was called Zhen Guan.

Having learned a lesson from the downfall of the Sui Dynasty, Taizong knew very well that unless the peasant classes were content, they could prove to be a very dangerous opposition under the leadership of anyone prepared to make a stand against the government. Taizong would remind himself of this frequently: the waters can both float and capsize a vessel.

Starting with political reconstruction of the state, he gave first priority to appointing people on their merits and seeking advice from able and worthy men, which became a major factor of his successful reign. He adopted and perfected the examination system that had been set up by the Sui. In this way men of talent, regardless of their class or background, had an opportunity to gain positions within the administration. Promotion of those who had achieved progress in office was decided. Possibly his most outstanding appointment was that of Wei Zheng, a former member of the Crown Prince's retinue. Wei Zheng had actually recommended the Crown Prince, brother of Li Shimin, to do away with Li Shimin only to be killed by the latter.

Tang Taizong worked out various systems and decrees favorable to the development of economy. He also advocated frugality, opposed corruption, and lightened the burden on the people. Policies adopted like: simplification of bureaucracy, strict control of expenses, elimination of corruption; inauguration of irrigation schemes, the enhancement of agricultural productivity, a fair system of taxation (Zu yong diao) coupled with the reintroduction of the land equalization system and reduction of corvee, and reformation of the penal code, eliminating the harsh laws of the Sui Dynasty.

Before long the Tang Empire presented a scene of general prosperity in which people lived and worked in such peace and contentment that no one would take any articles left by the wayside and doors were not bolted at night. This period was later referred to as "The Prosperity of Zhenguan".

After the domestic political and economic situation took favorable turn, Tang Taizong confidently started wars to unify border areas. He properly handled his relations with various nationalities by carrying out enlightened policies towards them, keeping their practices and customs unchanged, and appointing the former leaders there as local officials. Besides, he adopted the policy of cementing relations with rulers of minority nationalities in border areas by marrying daughters of the Han imperial family to them, which he did many times. For instance, he married Princess Wen Cheng to Songtsen Gammpo, leader of the Tubo, which is the ancestry of the Tibetan ethnic minority inhabited the Tibetan Plateau.

The unification and stability in border areas and the close relations among various nationalities helped promote economic and technical interchange between the Han nationality and ethnic minorities.

Furthermore, political, economic and cultural contacts between the Tang Empire and foreign countries became more and more frequent. Diplomatic envoys, merchants, scholars, artists, monks and priests from Asia, Europe and Africa came to visit the Tang Empire constantly. Tang Taizong set up special organizations and accommodations to receive them, and allowed them to stay over a long period of time and to marry Tang women. By means of exchange, a lot of strains of plants such as pepper, spinach, cardamom, tulip, etc. were brought to China in succession, and China's silk, porcelain, tea and paper were sold abroad in large quantities. It was also at this time that the paper-making technology, one of China's "Four Great Inventions", was introduced to foreign countries. Meanwhile, a large number of people of the Tang Empire visited other countries all over the world. It was from this time on that overseas Chinese began to be called "Tangren", i.e. Tang people.

Emperor Taizong ruled for twenty-three years and brought about the most prominent era of peace and prosperity in China's feudal society. For this, he is considered to be one of the most outstanding figures among China's feudal emperors. Under the management of Tang Taizong, the Tang Empire emerged as the most powerful feudal empire in the world.

Following his death in 649 at the age of fifty-three, he was buried in the Zhaoling Tomb located near today's Xi'an City in Shaanxi Province.