The US was so upset after the defeat in Vietnam that they failed to recognize that the war was mainly lost politically. The military strategy became the scapegoat despite the fact that the counterinsurgency was in fact quite successful during the final years. At Fort Bragg huge amount of documentation on how to conduct counterinsurgency was actually burned in the courtyard! The message was clear: Never ever get entangled in such a war again!
I often wondered why the lesson from Vietnam, and indeed those from Algeria that the French learned, seemed to have been lost when the US entered afghanistan and Iraq. Not until 2004 were the theories of counterinsurgency reinvoked, after many unnecessary lost years. Perhaps it is too late.