Vietnam was the right war, fought the wrong way.
And Vietnam was not it vain, it showed the Soviets that they were not going to take over country by country without stiff resistance. Imagine if we just let them have Vietnam without a fight?
The "domino theory" was valid.
Even if the domino theory was marginal and only Viet Nam fell it was enough. The Soviets wanted a warm water port in the Pacific from which to launch intelligence vessels and warships. The nuclear and space races were hot and getting hotter and they wanted to be able to not only monitor any and all of our progress in Micronesia where we had bases but to get a foothold in the area. The same way they were in Cuba and other areas around the globe where they could choke off major sea lanes or monitor our progress.
A point almost everyone ignores.
In later years the purpose of Nicaragua was that it was the next logical place for another canal across Latin America. Another choke point and bases from which they could send recon and bomber fights up our west coast which was too far from Cuba.
The Failure in Vietnam, by which I mean the collapse of South Vietnam, was owing to the fall of Nixon from power. The North Vietnamese won in a walk because (1) Congress cut off aid to the South and (2) the B-51s were held back when the North Vietnamese sent their armored columns into the South.
Creighton Abrams won the war; the Democrats kicked it away.