On the night I arrived in Irbil, Iraqi Kurdistan's capital, a Cuban band was playing in the restaurant at my hotel. The first song I heard it perform with a distinctive Caribbean verve was John Lennon’s “Imagine.” The plea for a world without killing seemed highly appropriate, if forlorn. Kurdistan is part of Mesopotamia, a cradle of civilization that witnessed such crucial developments in human history as the invention of the wheel, the first planting of cereal crops and the first...