In 1996 former US Secretary of Defence Robert McNamara was interviewed on CNN. He explained how the Domino Theory shaped John F. Kennedy’s thinking and whether Kennedy would have pursued the Vietnam War:

“The domino theory… was the primary factor motivating the actions of both the Kennedy and the Johnson administrations, without any qualification. It was put forward by President Eisenhower in 1954, very succinctly: If the West loses control of Vietnam, the security of the West will be in danger; “the dominoes will fall” in Eisenhower’s words. In a meeting between President Kennedy and President Eisenhower, on January 19the 1961 – the day before President Kennedy’s inauguration – the only foreign policy issue fully discussed was south-east Asia. Even today some question as to exactly what Eisenhower said, but it’s very clear that a minimum he said… that if necessary, to prevent the loss of Laos, and by implication Vietnam, Eisenhower would be prepared for the U.S. to act unilaterally – to intervene militarily.

And I think that this was fully accepted by President Kennedy and by those of us associated with him. And it was fully accepted by President Johnson when he succeeded as President. The loss of Vietnam would trigger the loss of Southeast Asia, and conceivably even the loss of India, and would strengthen the Chinese and the Soviet position across the world, weakening the security of Western Europe and weakening the security of North America. This was the way we viewed it; I’m not arguing (we viewed it) correctly – don’t misunderstand me – but that is the way we viewed it. …

There were three groups of individuals among his advisers. One group believed that the [Asian] situation was moving so well that we could make a statement that we’d begin withdrawals and complete them by the end of 1965. Another group believed that the situation wasn’t moving that well but that our mission was solely training and logistics. We’d been there long enough to complete the training, if the South Vietnamese were capable of absorbing it, and if we hadn’t proven successful… Kennedy listened to the debate, and finally sided with those who believed that either we had succeeded, or were succeeding, and therefore could begin our withdrawal… He made the decision (to begin withdrawing advisers) that day and he did announce it. It was highly contested…

Kennedy hadn’t said before he died whether, [if] faced with the loss of Vietnam, he would withdraw – but I believe today that had he faced that choice, he would have withdrawn rather than substitute US combat troops for Vietnamese forces to save South Vietnam. I think he would have concluded that US combat troops could not save Vietnam, if Vietnam troops couldn’t save it. That was the statement he in effect made publicly before his death.”