But the U.S. had an inbuilt capacity to accommodate and assimilate outsiders. The Founding Fathers knew they were creating what many now call a multicultural nation. They knew that the 13 colonies had diverse religious and ethnic origins—Puritan New England and Anglican Virginia, Quaker Pennsylvania and Dutch Reform New York. So the Framers of the Constitution provided that the U.S., unlike Britain, would have no religious test for federal office. And the drafters of the First Amendment provided that the federal government could make no law regarding an establishment of religion—which meant leaving alone established churches in the states (Massachusetts’ lasted until 1833). The work of assimilation was left to the states as well, and it is significant that the states with the largest immigrant inflow—Massachusetts and New York—were among the first to pioneer universal public schools, where children were encouraged to understand and respect a common civic culture…

But what really assimilated foreign-born Americans and reunited the American North and South was World War II. It was an annealing event, soldering together different American ores, in a way that nothing short of total war can do. Some 16 million Americans served in the military (the proportionate number today would be 38 million). Servicemen and defense workers were sent all over the nation. The military remained racially segregated, but the war experience raised the argument that people who risked their lives for the nation should be treated equally when they came home, an argument that persuaded Presidents Truman and Eisenhower to integrate the military, now the most racially equal part of society.

The war triggered a great surge of migration, with one-third of American blacks moving from the rural South to the urban North in the single generation between 1940 and 1965. Widely circulated black newspapers hailed the North as a promised land, where blacks would be free to pursue dreams and escape a nightmare. But in the middle 1960s, with the end of legal segregation (and the widespread adoption of air conditioning), the South was suddenly less of a nightmare and, with urban riots and continued residential segregation, the North less of a dream. The northward black migration suddenly ended—another phenomenon almost no one predicted.