The 2 biggest decisions that entangled America in Vietnam were made by Eisenhower and Johnson.
US Involvement in Vietnam: the first turning point
At the international conference convened to discuss French Indochina at Geneva in May 1954, the French exit was formalised. Vietnam was temporarily divided, with Ho Chi Minh in control of the north and the Emperor Bao Dai in control of the south. The Geneva Accords declared that there were to be nationwide elections leading to reunification of Vietnam in 1956. However, US intervention ensured that this temporary division was to last for more than 20 years.
The United States refused to sign the Geneva Accords and moved to defy them within weeks. Eisenhower's Secretary of State, John Foster Dulles, organised allies such as Britain in the South-east Asia Treaty Organisation (SEATO). The SEATO signatories agreed to protect South Vietnam, in defiance of the Geneva Accords, which had forbidden the Vietnamese from entering into foreign alliances or to allow foreign troops in Vietnam.
The Eisenhower administration encouraged Bao Dai to appoint Ngo Dinh Diem as his prime minister, and then proceeded to engage in nation building. Eisenhower and Dulles created a new state, in defiance (yet again) of the Geneva Accords and of what was known to be the will of the Vietnamese people. Eisenhower recorded in his memoirs that he knew that if there had been genuine democratic elections in Vietnam in 1956, Ho Chi Minh would have won around 80 per cent of the vote. In order to avoid a wholly Communist Vietnam, the US had sponsored an artificial political creation, the state of South Vietnam.
Within weeks of Geneva, Eisenhower arranged to help Diem set up South Vietnam. He sent General Lightning Joe Collins and created MAAG (the Military Assistance Advisory Group) to assist in the process. The US also helped and encouraged Diem to squeeze out Bao Dai.
The French exit meant that Eisenhower could have dropped Truman's commitment in Vietnam.