Your questions presuppose that they will have to. If this movement is smart they will enter negotiations with a 'rat administration and not have to fire a shot. They will bring in UN peacekeepers to secure the new borders and prevent "reprisals."
At that point we may as well give up on our Constitution because that would be in violation of that document. The UN cannot establish borders in our country. Also at that point, it would be all out civil war.