".the war has to extend to anywhere they are located and then see them made combat in-effective."
Hmmm - depends exactly WHERE they are located - if that is in a country with nuclear weapons the risks step change
I'd guess that no-one wanted another Korea with Chinese "volunteers" coming to the rescue of the NVA
And I'm not sure that history would have changed - at least not from where we currently stand - Thailand, Malaysia, the Philippines and Indonesia weren't domino's. Burma is still a dysfunctional shambles - and Laos, Cambodia & Vietnam are all run by tiny cliques