The 1924 act regulated immigration by allowing only a two-per-cent increase of any given ethnic group’s numbers each year. But, rather than using the most recent census, from 1920, to determine the immigration totals, the act referred to the 1890 census—a neat means of avoiding the swell of immigrants, designated as undesirable, from Southern and Eastern Europe, not to mention from Asia, who had arrived in the United States mostly in the intervening years. The policy was so defiantly and arrogantly racist that, as James Q. Whitman, a professor at Yale Law School, writes in “Hitler’s American Model,” it earned praise from Adolf Hitler. “The American Union categorically refuses immigration of unhealthy elements, and simply excludes the immigration of certain races,” Hitler wrote in “Mein Kampf.” This, he said, made the country a leader in preserving racial purity through immigration policy. The Johnson-Reed Act largely held sway for forty-one years, until, amid the democratizing ethos of the civil-rights era, immigration policy fully shed the racial engineering that had previously defined it. This is the world that Trump seems to be attempting to resurrect.