You gotta get enough people to accept the forced unity isn't working. Make them see splitting up is better than remaining in a powder keg.
Not really. Just have a rogue charismatic governor. The Texas governor defied the federal govt less than a year ago using their national guard over border disagreements. He could easily have taken it further if he wanted to.
@Formerly Black Trash is correct that modern govt is not prepared to fight in a populted area especially when with a peer with the same exact hardware as the federal military.
Personally I think the national guard thing was a mistake. They arent rural like fort sumter was during the civil war, but placed in populated areas. They should have been under the federal umbrella. If the Union ever collapses, it will be because of these decentralized bases with their own Jets, missiles, and anti-air equipment.