Had the United States not executed a coup d'etat on the Diem regime in the Republic of Vietnam, how long would it have taken the Diem regime to collapse all by itself? Additionally, how much would this shorten the war?
Assume that USA took NO action against Diem (and pretty much leave him as it is - not really):
- There would be no ground forces of US on Viet Nam. Granted, there are advisors and trainers, but that's about it. As far as I've heard, Diem accepted USA help, but he was against US Armed forces deployment in large scale.
- If rumors are correct (there are no official confirmation), Diem tried to pacificate with Ho Chi Minh. In 1963, he was given a cherry blossom tree (symbol for spring in northern area of Viet Nam) by HCM. It is unknown if HCM received anything in return (or before that). Best case scenario, both men would shake hands by 1965 (Yeah, Soviet, China and USA would be pissed, but who cares?).
- In a not-so-good scenario, it is quite likely that Diem regime can collapse by 1966 (if he showed no improvements) due to corruption and favoritism. I'm not sure if National Liberation Front (NLF, well known as VietCong, or VC) had enough support at this time (no US-backed coup and no US armed forces). But it is more likely to be execute by some of his generals.
Not sure about the unification...