It’s no mystery why images of unremitting violence spring to mind when one hears the deceptively simple term, “D-Day.” We’ve all seen — in photos, movies and old news reels, most of them routinely presented in suitably grim black-and-white — what happened on the beaches of Normandy (codenamed Omaha, Utah, Juno, Gold and Sword) as the Allies unleashed their historic assault against German defenses on June 6, 1944.