I have a friend who was over in Vietnam during Tet, and he told me, those over knew it was a disaster for the VC.
The problem was the VC wasn’t the main threat in Vietnam; the North Vietnamese Army was. They were the ones who smashed the gate with their tanks in 1975, not the Viet Cong.
The war was a just cause, but Eisenhower was right in determining there was no clear path to victory there. Too many people simply didn’t want us there, or the South Vietnamese governments we were supporting (and this was demonstrated by the failure of “Vietnamization”). When captured guerillas or NVA soldiers made clear how long they had personally been fighting, then asked the Americans interrogating them how long they’d be willing to stay in Vietnam, the point was clear - the American public would never stand for it.