Any law that the citizenry is, in large measure, unwilling to obey is a moot law, most especially if one of the modes of expressed disobedience is war. Sure, one can have an abstract discussion about it, but where's that discussion supposed to go, regardless of its resolution? If one or several states secede from the union, there're really only two options the union has for dealing with it: allow them to do so and move on, or forcibly return them to the union. Secession is an act, not mere talk and threats. Once that act is undertaken, the realm of options for what to do about it narrow to a binary set.