To keep the equities of the American Civil War straight, it's only necessary to remember a) there was no rebellion, b) there was no insurrection, c) the People, not their servant officeholders, are Sovereign and answer only to the ineffable Who Am, and d) who invaded whom.
Everything else is excuses and eyewash -- excuses for killing 620,000 men in the field, and almost a million people overall, over politics.
"The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence. "
Art4 Sec4 U.S. Constitution
I think you have a point.
The southron myth machine at work yet again. Yes, there was a rebellion. Yes there was an insurrection. Whatever the hell 'c' means. You don't invade your own country so nobody invaded anyone.