Currently theres still north and south
but its the
previous north and pacific coast states
vs
previous south and mountain/plain states
Alaska might be neutral(it's surrounded by canada and russia)
Hawaii will side with north
Florida will probably be a third theatre with the north half of it splitting and joining the (NCSA) New Confederate States of America.
Puerto Rico and Cuba might aid against the NCSA in that theatre
Canada and France might aid the Union in the north theatre
And China and Russia will aid the union in the western theatre.

This might seem unfair to the NCSA but one thing to know is that most Air Force and many naval/army/coast guard bases are in what would become the NCSA
And largest big factor is who supports the second amendment more.

As for nukes either everyone will know the radiation storm that could follow or somebody is going to be a dumbass and go indirectly suicidal by radiation.