With the partial exception of immigrant societies like the United States, mass democracy seems to depend on ethno-religious solidarity in a way that older forms of government did not. The most successful modern nation-states have often gained stability at the expense of diversity, driving out or even murdering their minorities on the road to peaceful coexistence with their neighbors.

Europe’s era of unexpected harmony, in particular, may have been made possible by the decades of expulsions and genocide that preceded it. As Jerry Z. Muller pointed out in a 2008 essay for Foreign Affairs, the horrors of the two world wars effectively rationalized the continent’s borders, replacing the old multi-ethnic empires with homogeneous nation-states, and eliminating — often all too literally — minority populations and polyglot regions. A decade of civil war and ethnic cleansing in the former Yugoslavia completed the process. “Whereas in 1900 there were many states in Europe without a single overwhelmingly dominant nationality,” Muller wrote, “by 2007 there were only two, and one of those, Belgium, was close to breaking up.”

Along the same lines, the developing world’s worst outbreaks of ethno-religious violence — in post-Saddam Iraq, or the Indian subcontinent after the demise of the British Raj — are often associated with transitions from dictatorships or monarchies to some sort of popular rule. And from Kashmir to the West Bank, Kurdistan to Congo, the globe’s enduring trouble spots are usually places where ethno-religious communities and political borders can’t be made to line up.