Li Shimin killed Li Jiancheng normally, because the whole world was basically Li Er's fight, and the concubines were all Li Er's. Li Jiancheng Li Yuanji's men could not turn the waves.
However, if Li Da killed Li Er, Li Er's men were not vegetarians, and would avenge Li Er and support Li Er's son to ascend to the throne. Moreover, Li Yuanji also has a tiger eye on the throne, without Li Er, it may be much easier to deal with Li Da, Li Si. And Lee was defenseless. Although Li Yuan is still there, will Datang go to split?
To quote @Huaxi Yinzhi published:
Killed Li Er, and then the Turks invaded Chang'an, in a hurry, Grand Prince Li advocated moving the capital, Li Yuan agreed, the imperial court went south, and the population crossed south, known as Eastern Tang. Li Yuan was the founding prince of the Western Tang Dynasty, and in the ninth year of the founding of the country, Li Jiancheng was the founding prince of the Eastern Tang Dynasty, which can also be called Ashina Jiancheng.
Li Er's response to the Turkic attack was simply a god of heaven! Personally, I believe that not only Li Jiancheng, but also other famous emperors in history cannot do better than him.
![Tang Yonghong: Will the cross-strait economy be decoupled? (2) The author, Tang Yonghong, deputy director, professor, and doctoral supervisor of the Taiwan Research Center of Xiamen University, see "Tang Yonghong: Will the Cross-Strait Economy Decouple?" （[fig]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)