There are two relevant lessons from the Civil War:
(1) The only people who can beat Americans in military terms is other Americans. And it is UGLY. Incredibly UGLY. We killed more of our own than anyone else has killed in any other war.
(2) The side with the larger functional industrial base (esp energy and manufacturing) WINS. Period. Everything else is too equal. That was the North last time. God forbid we end up there again, but if it does, it would be the South this time.
That said, we REALLY don’t want to go there. Sadly, I think many of the statists in DC and the moribund big Northern cities don’t care if we do. They really think that they will win and see it as a shortcut to their marixst state.
Ooops, I goofed - more Americans were killed in WWII than the Civil War (but not by a huge amount). Given the populations and technology involved, though, the surprise is how close the casualities were...