The Southerners believed they had a right to secede because it was only "Four score and Seven years ago" that the founders did exactly the same thing.
That "Declaration of Independence" thing asserted that "it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.
The only difference between the American War of Independence and the Civil war is that George III was not nearly so intent on destroying the Colonists as Lincoln was about stopping Southern independence.
Had George III been as fanatical as Lincoln, we would still be British. It is only a matter of luck and British acquiescence that we won.
Oh, and the fact that the Colonialists won their rebellion while the southern rebels lost theirs.