I have only dim memory of this story. Four 20th century states - South Carolina, Georgia, Alabama and Mississippi break away and form a separate nation. It is possible Louisiana was also one of the states, but I'm pretty sure Florida was not. They are popularly called some name that was made up of the initial letters of the breakaway states - but I don't recall exactly which order the states were in to form the name. Could have been, maybe, GAMS states or SMAG states, though neither of those rings a bell. This was likely a paperback book, likely decades ago. I do not recall what the issue(s) was that caused them to split. I think I remember that the break-off was by consent with the rest of the U.S., but that may be wrong. I also think I recall that the Gadsden Purchase played a role somehow - but I may be confusing that with yet another story.
I know it's not much to go on, but the people on SOL have, again and again, answered about stories with very few clues - and even some clues that turned out to be wrong.
Thanks, in advance, for your help.