There is no unilateral "Constitutional principle of secession" in 1860-61. Not anywhere to be seen by human eye in all of recorded history before that time. If you think there is, please post an example of any peaceful, legal, unilateral example of secession to use as a legal precedent for "the South's" ***claim*** that such a right existed.The immediate cause of the war was over the Constitutional principle of secession. The U.S. Government refused to recognize the right of the southern states to secede from the Union and the CSA asserted that right by seizing federal property. However, the issue of slavery was an important contributing factor that eventually led to the war.
I have been asking people online to present such an example for more than 20 years, yet no one ever has presented a verifiable example. To get you started, the very first example of "secession" I know of starts in 1869 when Iceland begins the process of separating from Denmark. It went off without a hitch (well, at least until Nazi Germany occupied Denmark), completely in accord with the law, over a 75 year period. Even that is not a workable example for "the South" -- Denmark and Iceland were in a Personal Union under a King, not a Federal State -- but if "the South" had tried a similar peaceful, reasonable approach like Iceland did, IMHO they probably could have convinced "the North" to go along with it.