The South could have retreated to the hills and fought a gorilla war until the North gave up and quit.
Plus it was much easier to “win” any war back then. After WW2 we put “rules” in place to make war unprofitable and unwinnable. And until “we” lose, those rules will be obeyed more of less except when we choose to not obey them since we are sort of exempt.
The whole world system is designed to keep us in power forever, and until someone defeats us it will remain that way.
So Lee should have gone ape, eh?
Maybe, but a lot had changed between 1776 and 1861. Nationalism, democracy, mass-circulation newspapers. Give up Philadelphia for Lancaster or York or Annapolis or Baltimore in the 1770s and people figure it's just the way things are and go on fighting. Surrender Richmond and take to the hills in the 1860s and people eventually figure that the war is over or the new country isn't worth fighting for anymore.
Guerilla war worked in Vietnam because the base territory in North Vietnam was never invaded by the US and RVN. After four or five years of guerrilla war in the American South it might have been the rebel armies who gave up, as happened in the our war with the Philippines insurgents. Or maybe the rebellion would have dried up to manageable banditry as happened with recent Latin American civil wars.