In 194, Cao Cao went to war with Tao Qian of Xu Province, because Tao's subordinate Zhang Kai had murdered Cao Cao's father Cao Song. Tao Qian received the support of Liu Bei and Gongsun Zan, but even then it seemed as if Cao Cao's superior forces would overrun Xu Province entirely. Cao Cao received word that Lü Bu had seized Yan Province in his absence, and accordingly he retreated, putting a halt to hostilities with Tao Qian for the time being. Tao Qian died in the same year, leaving his province to Liu Bei. A year later, in 195, Cao Cao managed to drive Lü Bu out of Yan Province. Lü Bu fled to Xu Province and was received by Liu Bei, and an uneasy alliance began between the two.