I guess a lot depends on which states secede, and if they have nuclear weapons in their possession in those state. 🙂
No state possesses nukes, as far as I know. All nukes are on federal property and under the control of federal minions.
There would be some interesting legal details - like what to do about native american nations created by treaty; citizenship. I assume borders would remain open, trade would remain without duties, and so on. Might well be agreement to abide by some but not all federal edicts.