The United States became involved in the Vietnam War to prevent southeast Asia from falling to communist forces. Although at first the United States only helped with money, material and advisers, after an attack on U.S. destroyers in the Gulf of Tonkin, it began more direct military intervention.
At first, the United States assisted France with its colonial war in Indochina as a result of their alliance in World War II. Later, President Dwight Eisenhower and then President John F. Kennedy were convinced that strengthening and supporting South Vietnam was a means of preventing communism from overrunning Asia. Kennedy in particular used the domino theory as justification, which posited that communist victory in one country would cause other countries to weaken and fall. Although reluctant to commit ground troops, by 1962, Kennedy had authorized around 9,000 U.S. military advisers in South Vietnam.
The war escalated after Kennedy was assassinated and Lyndon Johnson assumed the presidency. Two U.S. destroyers patrolling the Gulf of Tonkin in August 1964 were attacked by North Vietnamese torpedo boats. In retaliation, Johnson ordered North Vietnamese military targets bombed. Soon after, Congress passed the Gulf of Tonkin Resolution, which gave the president increased powers to escalate the war. The initiation of Operation Rolling Thunder, a massive aerial bombardment of North Vietnam, was followed by the arrival of the first American ground troops. By 1969, the United States had more than 550,000 military forces in Vietnam.