What we call “political polarization” is also a feature of the demise of the nation-state. “We Americans (or Peruvians, Ukrainians, etc)” is a historically new idea that needs to be cultivated and maintained. It was created to make the nation-state possible. Once this form of state withers away, all of its attributes disappear.
Instead of the “imagined community” of a nation, people swear allegiance to an imagined community of a political group. Instead of a flag, they have a mask. Instead of an anthem, they have a collection of slogans. Instead of national identity, they have a political or a tribal one.
In short, the people who kept saying that the end of nationalism would put an end to the animosities it unavoidably creates are not too smart.