During the First Indochina War, Thảo was a communist officer in the Vietminh and helped oversee various operations in the Mekong Delta in the far south, at one point commanding his future enemy Nguyễn Khánh, who briefly served the communist cause. After the French withdrawal and the partition of Vietnam, Thảo stayed in the south and made a show of renouncing communism. He became part of the military establishment in the anti-communist south and quickly rose through the ranks. Nominally Catholic, Thảo befriended Diệm's elder brother, Archbishop Pierre Martin Ngô Đình Thục—the devoutly Roman Catholic Ngô family strongly favored co-religionists and had great trust in Thảo, unaware that he was still loyal to the communists. He went on to serve as the chief of Bến Tre Province, and gained fame