And yet we are a nation of laws, good and bad. Hopefully the good eventually replace the bad, but in any case, legal does matter.
You take a very wide view of of Article I, which as far as I know wasn’t used to justify the war. It wasn’t until 1869 that succession became formally illegal. A bit late to apply it to the war.
Whatever larger Great Causes were tacked on later, Lee, his generals and his men were desperately trying to defend their homes and families in the face of aggressive invasion. Out numbered and out gunned, they did so quite well until 1863.
I am no apologist for the Confederacy or slavery. But if trying to defend your home, family, and country, even in a losing cause, is treason or criminal, then most of the soldiers in history have been traitors and criminals.
I think we have about worn this subject out.