In the aftermath of a prison riot in southern Guatemala, the eyes that peer out amid the tattoo of a skull covering half his face are contorted in pain as fellow inmates care for his bleeding shoulder. They are members of gangs, whose stories began in Los Angeles and then were exported to Central America. There the gangs grew, became more violent, and now they head back to the United States. Known as maras, the gangs formed in the 1980s, when immigrants fleeing brutal civil war in El Salvador settled in Los Angeles. To protect themselves from already established L.A....