I'm not so convinced that it was illegal, although I am surely not enough of a constitutional scholar to insist that I am right.
IMO, the War Between the States changed the entire complexion of the U.S.A., sending us on the road to a system of much greater federal authority. Ironically, this is what the South was trying to escape; it could be that if the Southern States hadn't seceded, that the State power they cherished would be relatively greater today, as it was earlier.
However, I am unconditionally glad that the Union and our great country were preserved. And I am also proud to be from the great state of Virginia.