The first horseman was named al-Qaeda in Manhattan, and it came as a message on September 11, 2001: that our meddling in the Middle East had sown rage and funded madness. We had meddled because of imperial ambition and because of oil, the black gold that fuelled most of our machines and our largest corporations and too many of our politicians. The second horseman came not quite four years later. It was named Katrina, and this one too delivered a warning.