O n a Friday afternoon in October, I got off the Metro at Waterfront Station in Washington, D.C. As in many of the subway stations in the nation’s capital, one typically exits via a long escalator, and as I rose toward the daylight, a National Guard squad was waiting at the top. A sergeant signaled to me urgently. “Sir, there’s an active shooter in the area,” the young man said, his languid Appalachian drawl only partly concealing his agitation. Pointing to a Safeway nearby, he said, “We advise