 Complete the following phrase:  As the Ngô Quyền march his forces to Đại La, Kiều Công Tiễn sent the envoys to Southern Han to petition for sending army to help himself. Emperor of Southern Han, Liu Yan, commissioned his son, Liu Hongcao to help Kiều Công Tiễn, he also led another army to supply his son. But Công Tiễn was executed before this army arrived. In the November 938, at the 