Didn't the civil war begin because a coalition of states decided to secede from the union? That was an organized effort and they created their own military. It is very possible a coalition of states might decide to exercise their state's rights and not enforce federal law. In fact that has already begun with sanctuary cities refusing to enforce immigration laws and states legalizing marijuana and other drugs. The state's might even block federal officers from coming into the states to enforce federal law. Didn't Montana do something similar to this regarding firearms manufactured and sold in the state. The real test will come when the federal government cuts off funding. Wait, the courts have decided they can't do that to sanctuary cities, right?