You remember the Americans had been gone for a couple of years when Vietnam fell, right?
American air support was guaranteeing the success of ARVN after Nixon had Vietnamized the war. They were holding their own then the dem congress cut all funding for the American pilots flying the American planes. Defeat was snatched from the jaws of potential success. Of course, just like in Ukraine, we were never attacked and we were lied into it. Do we need to walk the nuclear razors edge in Ukraine for another ten or fifteen years again before we leave these people alone?