Nobody should try to stop any state who wants to secede. This country was founded on a violent revolution, which means we aren't afraid to fight for what we want. Just make sure that you really want what you are getting.
If a state secedes, they lose the backing of the military, in all cases. If war comes to US soil, the states who seceded will be fighting the same war, without the immense power of our military (and also without our immense weapons ). Don't think for a minute that Iraq, or North Korea or anyone else who wants us all dead in a fiery mushroom cloud is going to give one damn about state lines.
If they lose the military, they also lose the National Guard, who is there to help in many instances like flooding, forest fires and the like. They also lose the financial backing of the National Guard.
If they are border states, they will have to have their own Border Patrol. Good luck building that fence now.
Passports will have to be surrendered.
Funding for highways, bridges, madical facilities, transportation, infrastructure and education will cease immediately.
Don't think your Congressman will be able to help, because he will be asked to leave Washington.
The FAA will no longer be available to you, so until you develop your own air traffic control system, no flights in or out of your state will allowed.
The FCC will no longer be available to you, so you are now responsible for licensing your own radio and TV stations, as well as telephone and internet.
Anyone receiving Social Security, Medicaid, Medicare, VA disability or SSDI will have to reapply. Congress will then have to decide whether or not to allow this aid to be made available to foreigners from the US government.
The Coast Guard will no longer be available to you if you are a coastal state, so you are responsible for monitoring your own illegal drug smuggling and foreign invaders.
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I wish I could take credit for this list, but cannot. It does, however, make you look at secession differently. Did me, anyway.