Why did we fight in Vietnam?Why did we fight in Vietnam?
The basic answer to this is that the US wanted to contain the spread of communism. The US felt that communism would spread to South Vietnam if the US did not fight on the side of the South.
The Vietnam War was part of the US policy of containment of communism. The US felt that the spread of communism was something that would be really bad for the world. For this reason, it was willing to fight to try to prevent the spread of that ideology. Vietnam was a major part of that fight.
Eventually, the US also fought in Vietnam to preserve its reputation. The US continued to fight there because it felt it had to in order to keep its credibility. If feared that its allies would not trust it if it did not fight to the last in places like Vietnam.
Despite the political arguments regarding the United States involvement in Vietnam, it, like all wars, was fought for economic reasons, namely, the acquisition of resources and markets. Off the coast of Vietnam lies one of the largest oil deposits on Earth (see http://www.oilcompanies.net/oil1.htm). Does the United States withdrawing the majority of its troops from the country by 1972, thus losing the war, have anything to do with the first Energy Crisis of 1973?
It was a continuation of a 20 year policy of containment, with the widespread belief that South Vietnam, Laos and Cambodia would fall "like dominoes", and that if communism wasn't fought and stopped in Asia, then it would eventually threaten mainland America. While this wasn't a very rational fear, it was widespread nonetheless, and many Americans actually volunteered to fight (although most were drafted) for this very reason.
The standard popular response is indeed the containment of Communism; the truth under the journalistic rhetoric is the acquisition of resources. It appears suspect that during the Cold War the US would commit so much military solely to Vietnam, when there were many other countries around the world teetering like dominoes to fall into the Communist camp.
I haven't actually ever heard the reasons given in #3 for justifying American involvement in Vietnam. Much more popular is the belief outlined in #4 regarding the policy of containment and the fear of Communist influence increasing in other countries that could in turn weaken America's position in the Cold War that was being waged around the world.