The son of Emperor Taizong was Tang Gaozong Li Zhi (628 ~ 683), and his territory was the largest in the Tang Dynasty.
There are still many achievements of Gao Zongchao, which completely solved the North Korean issue. Emperor Gaozong's Yonghui Law was based on Wu Law and Guan Zhen Law, and it was also the last law in the Tang Dynasty. Yonghui period continued the route of Zhenguan period. But in fact, this is not the credit of Emperor Taizong, but the assistance of two old ministers left by Emperor Taizong, Sun Chang Wuji and Chu Suiliang.
Tang Gaozong is in poor health, weak in character, and a typical mediocre person. This created an opportunity for Wu Zetian to usurp power. Originally, it was impossible to establish a father and concubine after the Song Dynasty, but in the Tang Dynasty, which did not emphasize etiquette, it did succeed. The consequence of this incident was that the eldest grandson and Chu later died at the hands of Wu Zetian. And Li Ji because in a word, this your majesty's family. After the Song Dynasty, it was attacked by historians. Blame him for not remonstrating to death. Li Zhi's performance in this matter is indecisive and timid.
It began to decline from the Tang Dynasty to the late Li Longji of Xuanzong of Tang Dynasty.