As was their perogative. That's what gets lost in the Civil War discussion. Nowhere does it say that a State can't leave the Union. Hell, Texas votes on it every two years. Those states declared their independence and left. Lincoln invaded a soveriegn country. It remains the only time in history that we invaded someone and kept the conquered territory.
Mass was on the way out a few years earlier but came to an understanding. Everyone missed the biggest point on why we have independent states, its how we have our religious freedom, each state basicly had their own seperate religions...