like some sort of earlier civil war, with the know nothings getting involved, and anglo-french intervention leads to the civil war ending up with some partial balkanisation - the south goes, obvs. but so does new york under a know nothing / democratic government and chicago gets annexed to britain, while france takes back new orleans. new england goes its own way after the war.
if we wanted to widen out the parallel, america endures the rest of the century under an ossifying 'national unity' presidency which is eventually overthrown somewhere near the century's end, and while an attempt to restore proper constitutional government stumbles, another revolutionary movement successfully seizes power in the 1940s, and manages to crush some of the old secessionists under foot.