Vietnam had been a French colony before it was occupied by the Japanese during World War Two. After World War Two it was returned to French control, but many Vietnamese people wanted independence. As a result, in the 1950s the French found themselves fighting a war against the Viet Minh - an organisation dedicated to removing foreign imperialist powers from Vietnam.
Worried about the spread of communism in South East Asia, the USA began to bankroll the French war effort in Vietnam. President Truman had said in 1947 that he was committed to halting the spread of communism. This idea formed part of his Truman Doctrine which shaped American politics during the early days of the Cold War.
In 1954, the French were finally defeated by the Viet Minh at the Battle of Dien Bien Phu. The French had tried to block a communist supply line and set up a defensive system at an army base in Dien Bien Phu. However, the communist Viet Minh forces trapped the soldiers inside the garrison for 56 days. The resulting media attention proved very embarrassing for the defeated French.
The outcome of this defeat was formalised in the Geneva Agreement of July 1954 and temporarily separated Vietnam into two zones: a northern zone to be governed by the Viet Minh, and a southern zone to be governed by an anti-communist government led by Ngo Dinh Diem. The Geneva Agreement spelt the end of French control of Vietnam, and the start of a major headache for the USA.
If you were going to pick a political leader to support, you probably wouldn’t pick Ngo Dinh Diem! As the leader of South Vietnam he was helped by America, and proved an unpopular choice with the Vietnamese people.