"No one answer"?

But there is one answer. Read Mark Twain's writings on US imperialism during Spainish-American War; those will enlighten you as to US intents in Viet Nam. See _Mark Twain and Anti-Imperialism_, Zwick.

Also note that Eisenhower said: "We cannot lost Viet Nam because we cannot lose the rubber, the oil, the manganese".

The purpose was not "liberty, democracy, freedom," etc.; it was $$$$$$$$$$$$$$$$$$$. If you don't think that is the fact, then look at the hard reality of US policy putting _S._ Vietnamese civilians into concentration camps. How did that "free" them? It didn't, except for those who died there, freed of life.