Across a frightened nation divided by politics and culture, a fragile harmony is ascendant, as Americans in small towns and large cities alike cry out in trembling unison: Hey, where did all these Californians come from? Talk of a “California Exodus” is sweeping the country—and so are anxieties about its effects on the rest of the West. In October, the Boise mayoral candidate Wayne Richey proposed at an election forum to build a $26 billion wall to keep out people moving from the Golden State. (His backup plan to stop the invasion of Boise? "Trash the place.”) A viral Wall...