In Saigon as of October 1961, Trueheart served as deputy chief of mission, the second-ranking U.S. diplomat in South Vietnam during what would become the final years of President Ngô Đình Diệm's rule, and during the initial buildup of U.S. military assistance to the Diem regime in its struggle against the Viet Cong.
During the spring and summer of 1963, as the Buddhist crisis intensified, Trueheart's analysis of the political and military situation diverged from that of the ambassador, Frederick Nolting.
As the ambassador vacationed, Trueheart warned of the possible liability to the United States of continuing to support Diem's government in South Vietnam, noted as "let[ting] loose the floodgates of doubt".
[2] Trueheart's position as the deputy chief of mission for the United States led to his involvement in the political turmoil which South Vietnam had had to embrace after the forced coup d'état of Emperor Bảo Đại in 1955.
[9] Polarisation between Diem and the Buddhists grew worse on June 11, 1963, when Thích Quảng Đức set himself alight in the process of self-immolation.