Li Deyu
An event of major historical controversy occurred in 831. Li Deyu was then the military governor of Xichuan. Xidamou (悉怛謀), the Tufan defender of Wei Prefecture (維州, in modern Ngawa Tibetan and Qiang Autonomous Prefecture, Sichuan), had surrendered Wei Prefecture to him. Li Deyu advocated accepted Xidamou's surrender and further use it as a base to launch a major attack on Tufan. Niu, citing the peace treaty that Tang had entered with Tufan and that a Tufan attack against the Chang'an region would be devastating, advised against accepting Xidamou. Emperor Wenzong agreed and ordered Li Deyu to return Wei Prefecture to Tufan. Tufan subsequently slaughtered Xidamou and his subordinates, leading to a public outcry. (For centuries following, up to today, historians disagree over whether Li Deyu or Niu was correct.)