In the early morning of July 2nd, 626, a bloody coup took place outside Xuanwu Gate of Li Tang Dynasty Palace, which was just nine years old. This is the famous "Xuanwumen Change" in the history of China. The king of Qin killed Prince Li and Li Yuanji, the king of Qi, and was appointed as the new prince, succeeded to the throne and was named Taizong.

The cause of this coup was that after Tang Gaozu Li Yuan ascended the throne, he made Li Jian the prince, Li Shimin the king of Qin and Li Yuanji the king of Qi. Among these three people, Li Shimin is not only brave and upright, but also outstanding. He is surrounded by a group of talented people who are both civil and military, such as well as Qin, so his prestige is extremely high. Li's exploits and personal prestige are inferior, so he was jealous, so he teamed up with his younger brother Li Yuanji, King of Qi, to crowd him out, and even poisoned his wine, trying every means to get rid of him. So Li Shimin decided to strike first, set an ambush at Xuanwu Gate, killed Cheng Jian and Yuanji, and finally ascended the throne.

It is true that to replace the Crown Prince, it is also right for Li to eradicate the future trouble, and it is also right for Li Yuanji to support the famous Crown Prince Li. In this dispute between the Crown Prince and the Crown Prince, no one is right or wrong. However, cannibalism will eventually leave a historical stain on such a wise monarch as Emperor Taizong.