 Complete the following phrase:  Dương Đình Nghệ ruled Tĩnh Hải for 6 years. In 937, He was assassinated by Kiều Công Tiễn, a military subject who had given his allegiance to the Southern Han state and seized power. Ngô Quyền, a former general and son-in-law of Dương Đình Nghệ, marched north from Ai to avenge the death of his patron. The pro-Southern Han Kiều Công Tiễn called Liu Yan for help. Liu Yan placed his own son, Liu Hongcao, in command of the expedition, granting him the titles Jinghai jiedushi and King of Jiao, sailed to the coast of Annam and headed inland up the Bạch Đằng River, a northern arm of the Red River delta, to confront Ngô Quyền. Liu Yan himself set out from Guangdong, following his sons fleet with additional forces. In late 938, Ngô Quyền 