US involvement in Vietnam was an outgrowth of the Truman Doctrine which prioritized a policy of the containment of Communism around the world. This policy had been successful in Greece and Turkey in 1952. It also had been applied in Korea in the early 1950s, preventing the entire peninsula from being taken over by Communist forces.

Vietnam had been divided along the 17th Parallel after the French withdrawal from the region in 1954. North Vietnam was led by the Communist regime of Ho Chi Minh. South Vietnam was run by the US-backed government of Ngo Dinh Diem. This was meant to be a temporary solution until broader elections were held. When the rule of Diem proved unpopular in South Vietnam, many South Vietnamese people threw their support behind Minh. While the United States was not a big fan of Diem either, they saw his regime as their best option to stop the spread of Communism in the country. As Communist elements gained strength and popularity in the south, the US felt obliged to send in military support to aid Diem.

The Domino Theory was popular among US foreign policy makers. They believed that if one nation fell to Communism, its neighbors would quickly follow. This was the case in Eastern Europe after WWII. There was concern that if Vietnam became fully Communist, the rest of Southeast Asia would succumb as well. Therefore, the US committed resources to fight the spread of Communism in Vietnam.