Meng Da was afraid that he would be punished for refusing to help
Guan Yu. At the time, his relationship with Liu Feng was also strained. Hence, in late August 220, he brought 4,000 soldiers with him and defected to
Liu Bei's rival
Cao Pi, who gave him a warm welcome. Meng Da then wrote a letter to Liu Feng, informing the latter that he was in grave danger, because someone close to Liu Bei had spoken ill of him, and he urged Liu Feng to surrender to Wei as well. However, Liu Feng ignored Meng Da's advice and returned to Chengdu, where he was executed by his adoptive father for failing to reinforce Guan Yu and failing to stop Meng Da from defecting. Cao Pi later ended the Eastern Han dynasty by forcing
Emperor Xian to abdicate the throne to him, after which he assumed the title of emperor and established the state of
Cao Wei, marking the start of the
Three Kingdoms period. In Wei, Meng Da received various important appointments and the title of a marquis. Cao Pi merged the three
commanderies of Fangling, Shangyong and Xicheng () to form a larger commandery, Xincheng (), and he appointed Meng Da as the Administrator () of Xincheng and tasked him with defending Wei's southwestern border. Liu Ye advised Cao Pi, "Meng Da is always looking out for rewards which are not rightfully his, and he is prone to schemes due to his talent. Thus, he will not be grateful to Your Excellency. Xincheng borders Sun Quan's and Liu Bei's territories. If the regional situation changes, it will create troubles for the realm."; Cao Pi ignored Liu. Meng Da became close friends with the Wei officials
Huan Jie and
Xiahou Shang. In 225, Li Hong (), a former Wei subject who surrendered to
Shu Han (a state founded by Liu Bei), came to meet Shu's chancellor
Zhuge Liang and the Shu officials
Jiang Wan and
Fei Shi. Li Hong told them about Meng Da's meeting with a person called Wang Chong (), who told him that Zhuge Liang urged Liu Bei to execute Meng Da's family after Meng defected to Wei, but Liu Bei refused. Meng Da also did not believe Wang Chong. ==Rebellion and death==