After Cao Cao took Yan province, his adviser Mao Jie told him that he should “serve the Son of Heaven in order to command those who do not submit” (奉天子以令不臣). Cao Cao took the advice. (Records of the Three Kingdoms) In 196, he moved the Han emperor’s court to Xu.
In the Longzhong Plan, Zhuge Liang told Liu Bei that Cao Cao could not be fought head-on. “Now Cao Cao has a million men, and he holds the Son of Heaven to command the lords.” The idiom 挟天子以令诸侯 is based on this line.