bleeding Kansas from coast to coast
But as I said, I don't think a economically gutted USA (a necessity to precipitate a real secession)would have the stomach or the resources to preserve a fractured union. They would sort out the situation as well as possible and probably wind up with 5-6 Republics on as good terms as possible.
I expect there would be some refugees moving both ways and there could be some localized violence by militias etc.