As is so often the case, the answer simply lies in decentralization.
If not being culturally united is the "problem" is that really the "answer"?
Nor can Buchanan possibly be referring to the US of the so-called Gilded Age. After all, during this period, the US was flooded with immigrants from a wide variety of backgrounds,
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The US has a long history of large riots related to immigration.
https://en.wikipedia.org/wiki/List_of_incidents_of_civil_unrest_in_the_United_States