In the course of humanity’s long, violent history, there was one brief, happy interlude, a decade or so on either side of 1900, when those fortunate enough to live in the more advanced parts of the planet were able to persuade themselves that barbarism had been banished from their culture, never to return. To those luckiest of men, the last true optimists, the barriers — psychological, political, and technological — that separated them from the savagery of the past were as reassuringly robust as the stout, solid bourgeois architecture then transforming their cities. This self-satisfaction we now know was pure [...]