The moon was still faintly visible over the garrison that houses the Mexican Army’s 20th Cavalry Regiment in Ciudad Juárez one cold January morning in 2009. Down the street, people huddled at corners, waiting for buses. Old cars rolled by noisily. But in front of the military camp, a family stood begging for the return of their missing son. Dressed in thick jackets, they held up fluorescent handwritten posters: Return Jaime safely; Jaime is just a student, not a delinquent; his mother is in despair.

Two nights earlier, a nineteen-yearold law student named Jaime Alejandro Irigoyen Flores had been forced from his home by a group of men in military uniforms. The next day, his panicked parents had made the rounds to police stations, government complexes, human rights offices, but all claimed to know nothing of Jaime’s whereabouts. Yet his family was sure he was inside the army base they now stood before. They feared he was being tortured, but at least that would mean he was alive.

The story might have seemed outrageous had it not fit a pattern that emerged in Juárez the previous year, when every day four or five bodies had turned up in a city of 1.3 million people. So far there was only one official explanation for the increase in violence: the country was ensnared in an escalating war among its six or seven largest drug cartels, and Juárez had become their most intense battlefield. By the end of 2008, Juárez’s annual homicide rate had quintupled, to 1,623 murders from 316 in 2007. But no tale of warring cartels seemed sufficient to explain those numbers, since it was also the year that President Felipe Calderón Hinojosa dispatched 2,500 soldiers and federal police to patrol the city's streets and comb at the cartels. Nor was it useful to conceive of Juárez as a place where anarchy and evil had simply been unleashed. Surely there was system driving the unprecedented slaughter, which, even for a city with a history of violence, now surpassed anything...