"No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility."
It will be funny as hell if Washington blockades the semiseceding States.
Well, they've been ignoring that one for a long while now, so there's no reason to believe they won't get away with bypassing the Constitution on other things as well.