While Thục was in Rome attending the second session of the Second Vatican Council, the 1963 South Vietnamese coup overthrew and assassinated his younger brother Ngô Đình Diệm, who was president of South Vietnam. Thục was unable to return to Vietnam and lived the rest of his life exiled in Italy, France, and the United States. During his exile, he was involved with Traditionalist Catholic movements and consecrated a number of bishops without the Vatican's approval for the Palmarian and Sedevacantist movements. As a result, he was excommunicated by the Holy See and later reconciled with the Vatican.