Like any American teenager, Brenda Paz spent much of her time with her friends. They would go to parties, listen to music, and show off their cars late into the night. But Brenda and her friends belonged to the Mara Salvatrucha--the MS-13--the most violent gang in America, and in addition to enjoying the things that all teenagers do, her friends were thieves, drug dealers, human traffickers, and murderers. A street gang that began in Los Angeles in...