This is quite an abstract subject in relation to Vietnam. Rather than me writing you a whole answer, ill skim over a few topics. I would focus on earlier president's involvements in Vietnam; beginning with Eisenhower and Kennedy who both increased troop numbers in Vietnam. I would also mention that the US had never lost a war. Both these factors would compound to fear of losing. The US was also the main opposer to communism and Johnson was a believer in the "Domino Theory" (Idea that if one country falls to communism the surrounding countries will too), so there was a fear in his administration that if Vietnam fell to communism Laos, Cambodia and Thailand would too (2 of which did). I would also mention the breaking of the "consensus" that existed in the US up until 1968, that was when the fear of ordinary americans to question government policy broke. I would then conclude with Robert McNamara's famous quote, that in Vietnam they were "fighting Nationalism, not communism". By this he meant that the North Vietnamese were fighting for a unified Vietnam free of foreign rule (French 1887-1954, US 1955-1975).
