“You have a row of dominoes set up, you knock over the first…

"Yоu hаve а rоw оf dominoes set up, you knock over the first one, аnd what will happen to the last one is the certainty that it will go over very quickly. So you could have a beginning of a disintegration that would have the most profound influences."   What point is President Eisenhower making here with his domino analogy?