I first met Michael Walzer about a decade ago. I was a nobody graduate student from Catalonia, finishing my dissertation. Walzer was a professor emeritus at the Institute for Advanced Study, in Princeton, and the former co-editor of the left-wing magazine Dissent, but he was most interesting to me as a philosopher—and, especially, as the author of “Just and Unjust Wars,” an attempt to delineate the rights and wrongs of warfare in both its causes and its conduct. In the nearly fifty years since it was published, no other book on the subject has matched its influence. I e-mailed him out of the blue, eager to discuss its ideas, and he invited me to his office. We talked about philosophy and politics on a walk in some nearby woods.