The April 1999 massacre at Columbine High School hurtled the United States into an era of mass shootings. Now, a quarter century later, the horrors of that day have become an almost regular facet of American life, and gun violence a record-smashing epidemic. Clad in both camouflage and Kevlar, 21st-century America is a bastion of freedom where workplaces require active shooter protocols, schools run lockdown drills, and the Second Amendment has become a religion unto itself.Guns are a uniquely American problem, but the U.S. wasn’t always like this. LONG SHADOW: IN GUNS WE...