And yet 100 years on, World War I offers a sobering reminder of man’s capacity for folly. When we say that war is “inconceivable,” is this a statement about what is possible in the world—or only about what our limited minds can conceive? In 1914, few could imagine slaughter on a scale that demanded a new category: world war. When war ended four years later, Europe lay in ruins: the kaiser gone, the Austro-Hungarian Empire dissolved, the Russian tsar overthrown by the Bolsheviks, France bled for a generation, and England shorn of its youth and treasure. A millennium in which Europe had been the political center of the world came to a crashing halt.