Most everyone knew then, and knows now, what the causes were.
But if for some reason they’ve missed it, all they have to do is read the Declaration of Independence, and the secession documents of the States of the South.
So am I correct that your objection isn’t to secession itself, which was Lincoln’s stated causus belli, but to slavery?
If that’s the case, then once again the Crown declared slaves emancipated during the Revolution. So do you then side with King George and emancipation? Or with the Americans and the continuation of slavery?
If your problem is secession itself, which was Lincoln’s position, how do you defend the Colonials seceding from Great Britain?