So I looked under the FAQ's and this wasnt there, at least not in the sense that I could get a clear answer? Also I hope this doesnt seem like a loaded question under the rules but Im just trying to understand.
If the US role in entering the Vietnam War was to stop the spread of communism, such as noted often by the Domino Theory and by Bernardito here then didnt the US achieve its goal and thus win the war? Considering that to win a war you need to achieve your goals, why Pyrrhic victories are a thing, and the US did achieve its goal of slowing/stopping the spread of communism in South East Asia and around the globe, didn't the US win their war in Vietnam? Maybe by fear the US would engage in a bloody war with them, or simply because by the mid 70's the spread of communism was already winding down, but not many more countries still became communist after the war in Vietnam if I'm right. Am I correct in this thinking or have I missed something? Isnt it possible that both North Vietnam and the US won their respective wars? As both achieved their goals.
Let's first explore the US goals.
These points are taken from a speech that US president Lyndon B. Johnson held at John Hopkin's University in April 7, 1965.
The United States was there to protect and uphold South Vietnam's democracy and independence. "Our objective is the independence of South Viet-Nam, and its freedom from attack. We want nothing for ourselves--only that the people of South Viet-Nam be allowed to guide their own country in their own way."
They were there to fight against communism and prevent communism from spreading throughout South-East Asia. "We are also there because there are great stakes in the balance. Let no one think for a moment that retreat from Viet-Nam would bring an end to conflict. The battle would be renewed in one country and then another. The central lesson of our time is that the appetite of aggression is never satisfied. To withdraw from one battlefield means only to prepare for the next."
The United States wanted to be seen as a reliable ally. If the US had left South Vietnam to its own devices and being run over by North Vietnam, what kind of ally would they have been? To put it simply: their credibility was at stake in LBJ's opinion. "There are those who wonder why we have a responsibility there. Well, we have it there for the same reason that we have a responsibility for the defense of Europe. World War II was fought in both Europe and Asia, and when it ended we found ourselves with continued responsibility for the defense of freedom."
The intervention in Vietnam has to be seen in the context of the Cold War. The United States had been involved in Vietnam since 1950, supporting the French armed forces with money and equipment at first and then taking a more active role after the French retreat in the newly formed South Vietnam. The US leadership genuinely believed that if South Vietnam fell to North Vietnam, then communism would spread to South-East Asia and that countries would fall in quick succession - like dominoes. But if the US could stop the tide by containing communism before it spread, then they were doing a favor to the whole "free world" in their eyes.
In retrospect, like you point out, communism did not spread to the shores of California and it was limited to countries in the immediate vicinity who had been affected by the war in Vietnam: Cambodia and Laos. There were attempts by communist insurgents in Thailand to do the same in a conflict that lasted a decade longer but in which they were defeated. However, we can say this with hindsight and that's perhaps the blessing of being an historian. Seeing out of contemporary Cold War eyes, this was a major threat and belief which shouldn't be discounted. However, in the perspective of South-East Asia, Cambodia and Laos did fall.
Then we have South Vietnam. South Vietnam did not remain as an independent, democratic nation. In fact, it wasn't democratic to begin with but it was most certainly independent. It would become neither as it fell to North Vietnamese in 1975. Keeping South Vietnam independent had been the most important goal for the US, the reason to why they had spent almost 20 years in southern Vietnam. The fact that the North Vietnam, in their excellent propaganda campaigns, painted out the US being fought against by peasants with nothing but old bolt-action rifles and rocks made the impact of the US leaving South Vietnam all so much greater and humiliating for the United States.
All in all, no. The United States did not achieve its objectives in the war. They did not achieve any major goals (as outlined above) or anything resembling to military goals.
Were these objectives ever really obtainable? If the US had from the very start chosen to go with a completely different military strategy, favoring a time consuming pacification effort over a (supposed) short strategy of attrition then things might have gone differently. But both the US civilian and military leadership was completely incapable of adapting to the circumstances due to their rigid thinking.
Oh and Sorry if it seems like I called out Bernardito, I was just using that answer as my basis