Gannan Tibetan Autonomous Prefecture
By summer of 649, Emperor Taizong was seriously ill—with some believing that his illness was caused by his taking of pills given to him by alchemists. Believing Li Shiji to be capable but fearing that he would not be submissive to Li Zhi, he demoted Li Shiji out of the capital to be the commandant at remote Die Prefecture (, roughly modern Gannan Tibetan Autonomous Prefecture, Gansu), with instructions to Li Zhi that if Li Shiji hesitated, to execute him immediately, and if he did not, to recall him after Emperor Taizong's death and make him chancellor. Li Shiji, when receiving the order and realizing that his life was at stake, immediately departed for Die Prefecture. (After Emperor Taizong's death, Li Zhi would indeed recall Li Shiji and make him chancellor.) Soon thereafter, Emperor Taizong, after entrusting Li Zhi to Zhangsun Wuji and Chu Suiliang, died at his summer palace Cuiwei Palace (). His death was initially kept a secret, and three days later, after his casket had been returned to Chang'an, his death was announced, and Li Zhi took the throne as Emperor Gaozong.