In 937, Dương Đình Nghệ, the ruler of Tĩnh Hải quân, was killed by his general Kiều Công Tiễn. Ngô Quyền, who was Nghệ's son-in-law, mobilized his army against Tiễn. Tiễn requested help from Southern Han. Liu Yan decided to annex Tĩnh Hải quân, so he granted Liu Hongcao the title Prince of Jiao (交王, Giao vương, "Prince of Giao Chỉ"), and dispatched him to Vietnam. Liu Yan himself commanded a follow-up army. In the next year, the army of Southern Han was defeated by Ngô Quyền in Bạch Đằng River, and Hongcao was killed in the battlefield. Hearing the news, Liu Yan cried bitterly and withdrew his own fleet.