You know, this question gets asked a lot in the 3/4-drunk stage of cocktail parties. You are supposed to answer, “No, of course not!!!” and go refresh your drink.
Given that a direct consequence of South Vietnam’s collapse was the slaughter of at least 1 million South Vietnamese, plus another 1 million in Cambodia and Laos (we’ll never know the exact figures), I think we could say that for the time we were in Vietnam and keeping the North Vietnamese at bay, we prolonged the lives of 2 million people by a decade or so. That seems to me to be an absolute good.
I don’t know the number of Vietnamese immigants currently in the US, but I think that that population is far happier and more prosperous than they would have been in the Vietnamese worker’s paradise, especially since the ones that fled were for the most part, the most likely to be rounded up and killed.
While our presence in Vietnam may ultimately have been a failure, it did send the message to China and Russia that we would not just sit back and watch while they destabilzed every government they could and encouraged their puppet states to start brushfire wars.
And yes, pho shops.