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 southern regime 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.
Following the French departure and the separation of Vietnam, Thảo stayed in the south and pretended to renounce communism. He became a part of the military elite in the anti-communist southern government and quickly rose through the ranks. Although claiming Catholicism, 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.