Having abandoned the Communist-led Việt Minh resistance to join the Vietnamese National Army, the predecessor of the ARVN, Mau rose to be head of military security under Diệm. At that time a colonel with no troops to command, Mậu was nevertheless an important member of the conspiracy due to his liaisons with a wide number of officers, which allowed him to recruit widely for coup participants. He initially tried to organize a coup group himself with Colonel Phạm Ngọc Thảo, an undetected communist agent bent on maximising infighting, and disillusioned intelligence director Trần Kim Tuyến mainly consisting of mid-level officers. Later this group was integrated into the main plot led by a group of generals; Mậu had helped to liaise between some of these generals. He also concocted false data to convince Diệm to send the ARVN Special Forces—mainly used to defend Diệm and his family from coups in Saigon—into the countryside to battle a non-existent large-scale communist attack. The coup was successful and Diệm was captured and executed.