Regarding “causes of the war”, there was plenty of hypocrisy on both sides. Slavery was certainly a major factor as stated in a number of the sothern states own declarations.
Moreover, there was political support in the south to forcibly annex places like Cuba in odrer to get more pro-slavery senators. Forcible annexation and allowance of secession seem to be mutually exclusive to me.
That said, I believe that the individual states retained enough sovereignty to secede, regardless of the consistency or value of their underlying reasons. This should have been addressed in the original Constitution.
Secession is legal, by federal amendment, by federal legislation, or even by successful court case.
The southern insurrection had none of those. The temporary union of the first continental congress became a perpetual union by the Articles of Confederation, and in turn was made more perfect by the current constitution.