>>when Walter Cronkite said on the air that he believed the war was unwinnable

He wasn't wrong. My take on it at the time was that it was a farmers' uprising against rich landowners that we were on the wrong side of. The only way to win was to get these farmers out from under the thumb of those landowners - and we weren't about to do that. I had a good friend who as a provincial representative (ProvRep) there, was trying to improve the situation for the farmers - with some small successes but hampered by the overall situation. That was the Viet Cong war, which we did win, in the sense of, essentially, wiping them out. Laid on top of that was North Vietnam's armed forces - and we were unable to block the border.

At the time, I thought the whole "Domino Theory" was utter nonsense, but I now see it as something that grew out of the Bretton Woods Accords (US guarantees security of signatories). If we hadn't gone in, that would have brought that guarantee into question - at the height of the Cold War. At least, I think that's how JFK got levered into it.