What exactly is the federal government going to do about it though? I think using the military to suppress a state that was attempting a peaceful secession would be very hard for the government to justify. It's a possibility but I think the probability is low that US troops would be deployed on US soil to prevent a state seceding. Plus I expect the federal government to have very major financial problems which will limit its ability to act.
Few people in 1982 would have predicted that the USSR would allow its constituent republics to secede peacefully within a decade.
It is settled legally, that the states do not have the authority to secede, they tried during the Civil War. Many people thought that states could leave the union at that time. However the precedent set by Lincoln's actions are unchallenged now by the legal establishment.
Anyway, the procedure would go like this:
then
2a. Federal government challenges legality of secession in courts.
3a. Supreme court declares the secession unconstitutional.
Or:
2b. Federal government charges rebels with Treason.
3b. Federal government ar...
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.