There are not many moments in history when it is possible to worry that the world has become too happy for its own good. One such moment came in Europe during the late nineteenth century, when the Napoleonic Wars had receded into the distance and the First World War was still hidden over the horizon. For a brief period, it became possible to believe that the West was headed for a condition of permanent peace; that technology, democracy, and globalization were driving a virtuous circle that no atavistic violence could disrupt. This vision never came very close to becoming a...