The question of whether or not secession was legal or not was not addressed in the Constitution except under the 'all other rights clause' granted under the 9th and 10th Amendments since the anti-secession clause of the Articles of Confederation were not included in the new Constitiution. But servitude in all of it various forms, indenture servants, apprentise, bond servants, ophans, and wards of the state, and including slaves and the ownership of Property were addressed specifically in the Constitution.
This country prides itself on its Independence and Freedom, yet would not allow 11 States to form their own country and would force those states to remain a part of the Union, no matter how desireable that may be, against their will, and no matter what the reason for their wishing to leave that Union. We states, and the people therein, are now defacto prisoners of a central federal Government and its military might. THAT was not the intent off the framers of that Constitution.
Since the southern states were seceeding they were not attempting to overthrow the United States Government, they were mearly chosing their independence to no longer be a part of the country and that Government. Therefore they were not in rebellion then and never were at any later date.
So to me the war was about "Preserving the Union" for the North, and "Independence" for the South. And of the two "Independence" was the greater issue. "Preserving the Union" was just a metaphor for the repressing a people who wished for their own independence government. "Freeing the slaves" was anti-Constitutional and the President of the United States did not have the authority to abolish the practise in his own country let alone in someone else country, nor did he in his own country until December 1865 when the issue became an Amendment to that Constitution and "Legally" ended most forms of the practise.