This infantry and armor Vietnam combat vet would like to issue a forlorn welcome to the club to all of my contemporary veteran comrades. But in the case of South Vietnam, they DID fight fairly effectively for over 3 years after the last US combat units were withdrawn, until their logistical train was cut due to Democrat (is there a pattern here?) machinations in the wake of Watergate and Richard Nixon. Remember, a black clad guerilla Army did NOT seize and flatten the gates of the Presidential palace in Saigon. It was T-54, and T-55/T-59 tanks lavishly supplied by the Soviet Union and Red China, It is very difficult to defend against an armor led blitzkrieg when your artillery firing batteries are rationed to 10 rounds a day, and your aircraft can’t fly due to lack of fuel and spare parts.
Now, we have a senile old DEMONRAT goat as POTUS, with an utterly inept and inadequate plan for withdrawal from Afghanistan, as he pitifully begs the Taliban to not damage US soil in the form of the US Embassy whenever they seize Kabul, as they leave a treasure trove of first line US equipment for them to seize.