Not long after his 19th birthday, David Vallat, a native Frenchman born to a secular family, converted to Islam. He was having an “existential crisis” and his new faith helped him curb his juvenile bêtises, or “bad behavior,” he told me. Few questioned his choice to convert either then or later when he joined the French army in 1992, to, as he saw it, protect the Bosnian Muslims in Yugoslavia. He would not “stand idly by” in the face of another genocide. Yet the war was a shock: After escaping death twice in three days, Vallat considered returning home—“as a coward,” he said.