Yeah 1868.
The supreme court acts slowly. That was the first time the court dealt with seccesion as far as I can find. sue me. Far be it from me to question the authority of the Supreme Court on constitutional issues, or the state legislature of slave holding states as to why the seceeded.
However, the state legislatures, by voting for secession, conspired against the established federal government.
This was an "act or instance of revolting against civil authority or an established government"
It can also be characterised as "organized opposition to authority; a conflict in which one faction tries to wrest control from another."
These are dictionary definitions of insurrection.
The fact that congress has the constitutional power to put down insurrection should say something about a state right to seceed. Perhaps if that secession was disorganized, but that would fall under the federal responsibility to ensure domestic tranquility wouldn't it?