I was looking through some old posts for import, and found this from 2007. It's by an old professor and friend of mine, and is as relevant today as it was then.
And there is this, finally, to say about America’s avoidable debacle in Vietnam: something very much like it could happen again. Not in the same place, assuredly, and not in the same way, but potentially with equally destructive results. This is the central lesson of the war. The continued primacy of the executive branch in foreign affairs – and within that branch of a few individuals, to the exclusion of the bureaucracy — together with the eternal temptation of politicians to emphasize short-term personal advantage over long-term national interests, ensures that the potential will exist… . If future Vietnams are to be prevented, the American people and their representatives in Congress will have to meet their responsibilities no less than those who make the ultimate decisions. Otherwise, American soldiers will again be asked to kill and be killed, and their compatriots will again determine, afterward, that there was no good reason why.
— Fredrik Logevall in Choosing War: The Lost Chance for Peace and the Escalation of War in Vietnam