All our fashionable blather about “diversity” notwithstanding, we live in an age of ethnic disaggregation. Czechs and Slovaks, Serbs and Croats, Greek and Turkish Cypriots, Abkhazians and Ossetians and Georgians, have all separated after centuries of cohabitation. The Flemish and Walloons of Belgium look set fair to do the same. The Jews are long gone from Arabia and Persia, the Saxons have mostly decamped from Transylvania, the Nepalese are leaving Bhutan, and the Bantus want out from Somalia. The Protestants and Catholics of Northern Ireland are less mixed now than they were a hundred years ago, and, in the week of Barack Obama’s inauguration, Reuters ran a report headlined “U.S. School Segregation on the Rise.” Jerry Z. Muller’s striking article “Us and Them: The Enduring Power of Ethnic Nationalism” in the Spring 2008 issue of Foreign Affairs argued that this ethnic disaggregation, far from being a ...