The Johnson Administration based its policies on a number of assumptions. Let us look at two of the assumptions that had the most to do with the Vietnam War.
First, the Johnson Administration, like all American administrations of the Cold War, assumed that the communists were trying to take over the world. They assumed that any communist advance, no matter how far from the United States, was a danger to the US because it was one more step towards the communist takeover of the world. This made conflicts in such places as Vietnam important to the US.
Second, the Johnson Administration assumed that all communist countries were essentially controlled by the USSR. They believed, for example, that a communist Vietnam would be a puppet state of the Soviet Union. Therefore, they worried more about a communist takeover of Vietnam than they would have if they had thought Vietnam would be truly independent.