When I was getting ready to take my Ph.D. exams forty years ago, I had a meeting with my advisor, Raymond Sontag. What, he wondered, should he examine me on? “Why don’t you ask me something about the origins of the First World War?” I said. “I think I understand that now.” His reply was devastating: “Oh really? I’ve been studying it for fifty years and I still don’t understand it.” But over the years I’ve come to feel the same way. The whole question of what caused that war, for me at least, remains deeply puzzling. To be sure, we’re still learning new and important things, even about what happened during the July Crisis in 1914. But with every new insight, new problems come into focus, and ultimate answers remain as elusive as ever. In fact, the deeper you go into the issue, the more puzzling it becomes—or at least that’s been my own experience in grappling with this particular historical problem.