That legacy does not fall on Kennedy.
American foreign policy after World War II was based on the goal of containing Communism and the assumptions of the so-called "domino theory"—if one country fell to Communism, the surrounding countries would fall, like dominoes. In response to that threat, the Southeast Asia Treaty Organization (SEATO) was formed in 1955 to prevent Communist expansion, and President Dwight D. Eisenhower sent some 700 military personnel as well as military and economic aid to the government of South Vietnam. The effort was foundering when John F. Kennedy became president.
Corruption, religious differences, and mounting successes by the Vietcong guerrillas weakened the South Vietnamese government of Ngo Dinh Diem. Diem was Catholic, and public protests over the repression of Buddhists threatened the stability of his regime. Kennedy accelerated the flow of American aid and gradually increased U.S. military advisers to more than 16,000. At the same time, he pressed the Diem government to clean house and institute long-overdue political and economic reforms.
The situation did not improve. In September of 1963, President Kennedy declared in an interview, "In the final analysis, it is their war. They are the ones who have to win it or lose it. We can help them, we can give them equipment, we can send our men out there as advisers, but they have to win it, the people of Vietnam, against the Communists. . . . But I don't agree with those who say we should withdraw. That would be a great mistake. . . . [The United States] made this effort to defend Europe. Now Europe is quite secure. We also have to participate—we may not like it—in the defense of Asia."
A few weeks later, on November 1, 1963, the South Vietnamese government was overthrown. The coup had the tacit approval of the Kennedy administration. President Diem was assassinated, after refusing an American offer of safety if he agreed to resign.
In the final weeks of his life, President Kennedy wrestled with the future of the United States' commitment in Vietnam. Whether he would have increased military involvement or negotiated a withdrawal of military personnel still remains hotly debated among historians and officials who served in the administrations of President Kennedy and President Lyndon B. Johnson.
United States military aid to Vietnam increased during 1964. By 1965, President Johnson authorized U.S. troops to begin military offensives and started the systematic bombing of North Vietnam. By 1968, the number of U.S. forces surpassed 500,000. During that year's presidential campaign, Americans were deeply divided by the deteriorating military and political situation in Vietnam.