On Monday night, President Trump declared himself a nationalist. At a rally in Houston, he distinguished nationalists from “globalists,” people who, according to Trump, care more about the world than about their own tribe. Nationalism has a fascist pedigree, and Trump has a history of using race, religion, and ethnicity to turn the people he sees as real Americans—people who look like him—against those who have “Arab” or “Mexican heritage.” But violent nationalism, the kind that devoured Europe a century ago, couldn’t happen in America today, could it?