Nguyen Van Thieu, president of Vietnam from 1967 to 1975, started his career as a village chief for the Vietminh. In the late 1940s he joined the French colonial army and after Dienbienphu was sent to the United States to receive military training. As an army Colonel, he directed the attack to the presidential palace during the coup against Diem in 1963.

January 5, 1973 letter from Nixon to President Thieu
From Tom Dier's Vietnam Page
http://greene.xtn.net/~wingman/docs/nixon1.htm

With the negotiations in Paris reaching a point of agreement, Nixon exerted very strong pressure on Thieu to accept the eventual settlement. In this letter, Nixon warns the South Vietnamese leader that "[t]he gravest consequence would then ensue if your government chose to reject the agreement[...]."