李元吉
In summer 626, with Eastern Tujue attacking again, Emperor Gaozu commissioned another son, Li Yuanji the Prince of Qi, who was part of Li Jiancheng's faction, to lead troops against Eastern Tujue, and Li Yi was assigned to be under Li Yuanji's command. However, at this time, Li Shimin, apparently fearful that Li Jiancheng and Li Yuanji were about to kill him, acted first, ambushing Li Jiancheng and Li Yuanji and killing them at the Incident at Xuanwu Gate, and then effectively forced Emperor Gaozu to first create him crown prince, and then two months later yield the throne to him (as Emperor Taizong).