Czechslovakia, Soviet Union (arguable), Breakup of the Benelux countries, just to name a few current ones.
Hi, Will 001
In my humble opinion, the USA cannot be partitioned (for now at least) for the same reason that Switzerland cannot be. The country is founded on ideological grounds, not nationalistic, historic, ethnic, genetic or lingual. These aspects play a role, sure, but they are trumped by the American Idea. If however, the Americans choose to place genetics, race, or whatever else over ideology, then partitioning is inevitable. But I think that is far from happening, despite what the alt-right and the leftists have been doing.
I would be far more concerned about France and Germany in your stead.