IMPERIAL, Calif. — Ten minutes into arrant mayhem in this town near the Mexican border, and the gunman, a disgruntled Iraq war veteran, has already taken out two people, one slumped in his desk, the other covered in blood on the floor. The responding officers — eight teenage boys and girls, the youngest 14 — face tripwire, a thin cloud of poisonous gas and loud shots — BAM! BAM! — fired from behind a flimsy wall. They move quickly, pellet guns drawn and masks affixed. “United States Border Patrol! Put your hands up!” screams one in a voice cracking with...
