like dig, i wonder what the basis for your constitutional argument actually is.
it seems to me the south forced the issue. it might not have required a war had they not seceded.
personally i think they might have been allowed to secede. but they went about it the wrong way. see scotland's current discussion about secession from the united kingdom for a different, more gradual approach.
for myself, i think lincoln had a duty to keep the untied states united, unless there was specific language to the effect that the nation is divisible.
as regards slavery - i think it was in the bundle of causes that developed into the south's decision to secede. was the tea tax the cause of the war of independence? - no. it was one of a bundle of causes that collectively resulted in a declaration of independence.
but in the final analysis, the act of secession itself caused the civil war. just as the declaration of independence forced the war with great britain.