Lee was clearly a sociopath (It takes a good sociopath to effectively send men into battle) that really, REALLY blew it at Gettysburg. Multiple mistakes were made.
All that said, I side with the south’s states rights sentiment, but I’m glad the union was preserved. It would be a very different continent and world had the south succeeded. We’d be like France and Germany. And come 1940 that could have been ugly.
If Lee was a sociopath, so was every other general. That’s what command is - sending men into battle knowing some of them will die.
As for 1940....had the US not intervened in the first world war and tipped the balance in the Entente’s favor, it almost certainly comes to a negotiated settlement. Both sides were exhausted. No Versailles and no Triannon Treaties and Imperial Germany is left intact with the constitutional monarchy still intact. Hitler then is politically impossible. The Hapsburg Empire remains intact. An enormous power vacuum is not created in Central/Eastern Europe that Germany or the Soviet Union was guaranteed to fill. WWII simply does not happen - nor does the holocaust.
So if the Southern states had left, would the remaining rump US have intervened in WWI? It would have been nowhere near the power it was with about 1/3rd of the land and population gone. It might’ve stuck to a more modest foreign policy as it had generally had prior to its entry into WWI.