4 Answers

I'm not sure we in the USA ever really knew. Or at least knew why we were there. 

Read about it here >> http://en.wikipedia.org/wiki/Vietnam_War 

My father always said it was because the US arms dealers  wanted to test the latest arms and chemicals, and make money, which they did. It was a waste of human iives and misery. It's the same with Afghanistan, there is no reason to be there. Shame, shame, shame. Those people are never going to change their way of thinking.

The French had allowed US oil companies to drill test wells over South Viet-Nam Before the rise of the Khmer Rouge. The result of their testing showed significant oil deposits. The 1954 decision in congress provided an answer to the rise of the Communists in SE Asia that called for the US to engage in small wars to avert world wide wars. Since Kennedy was un-willing to stay with the plan that would have started a nuclear war with the Soviet Union by not contacting the Soviets and just going for the Bay of Pigs invasion his contact averted the disaster. Kennedy had just sent military advisors to Viet-Nam and made the speech regarding "Going to the moon”. The CIA saw this as a considerable opportunity in that JFKs assassination would immortalize his leadership into space and thus guarentee an unending flow of money for the CIA and NASA.   Lyndon Johnson pressed for an increased military presence in Viet-nam after JFK was killed and pressed congress onward on-ward in JFKs memory. Johnson was from Texas and had close ties in the oil industry. 

The American government was paranoid over communist countries. Partly because we used the bomb in Japan and Russia had no trust in us at all. I am not placing any kind of judgment over the bomb I was not alive at the time but the cold war may have been the biggest war that America has ever lost even though it is thought they won it. The money that was spread around the world by America even to this day will play a big part in the destruction of many counties if not the entire world. In America, they talk about how to cut spending but you will NEVER hear them say they will cut spending by stopping the pipeline of money that goes to every corner of this planet. The old saying holds true you cannot buy love or happiness.