In Jerusalem, Israeli and Jordanian militias patrolled a fortified, impassable Green Line from 1948 until 1967. In Nicosia, two walls and a buffer zone have segregated Turkish and Greek Cypriots since 1963. In Belfast, "peaceline" barricades have separated working-class Catholics and Protestants since 1969. In Beirut, civil war from 1974 until 1990 turned a cosmopolitan city into a lethal patchwork of ethnic enclaves. In Mostar, the Croatian and...