America resumed: 9/11 remembered

If asked to describe the cultural legacy of World War One, you might cite Hemingway’s A Farewell to Arms or George M. Cohan’s rousing but now forgotten “Over There.” Or perhaps the poignant battle monuments of the American Expeditionary Force, of which Paul Cret’s temple at Chateau Thierry is the loveliest. But these items are tangible, and the most vital cultural legacy of any war—or any great national trauma, for that matter—is intangible. It is the comprehensive way it changes our shared attitudes and assumptions, our collective sensibility.

Changes in the collective sensibility, being invisible, usually do not reveal themselves until they are expressed in action. The whip can crack before anyone realizes that it was coiling, and so it was at the end of World War I with Prohibition. In 1919 two separate forces—a wartime mood of urgency and a newfound bitterness toward...