The most compelling theory, however, is even simpler than that, chalking up Juarez's drop in crime to good, old-fashioned police work. The murder rate began dropping about a year and a half ago, around the arrival of a new police chief named Julian Leyzoala, who brought with him a back-to-basics, broken-windows approach to policing. A former military colonel who studied at Mexico's equivalent of West Point, Leyzoala came up in Tijuana, where he served as warden of prisons and later as police chief.

At his office in Juarez, in a police station that feels more like a military compound, Leyzoala showed me a series of charts tracking the city's crime rates. "In one month in 2010, there were 259 murders," he said. "Last month, we had 29." When I asked him why, he smiled. His manner is casual and relaxed, but the deep bags under his eyes suggest the stress of his current assignment. For safety, his family lives in the U.S., and he only sees them once every four months. "In Mexico, the police chief is usually someone who likes guns and has friends in high places," he said. "But police work is not for amateurs. It is for professionals, and I am a professional."

When Leyzoala arrived in Juarez, the city was virtually lawless. Convoys of Suburbans and Escalades rumbled through town, voices broke into radio traffic taunting police and feeding them misinformation, and an estimated one-third of the police force worked for the cartels as drivers, lookouts, and hitmen who did their killing in uniform.

On one of Leyzoala's first days, a banner surfaced over a local highway: "Welcome to Juarez, Leyzoala. Tonight we kill you." He survived the night, but the next morning brought a fresh threat: a police officer would die every day until he quit. By dawn, one of his cops was found dead in the street, and another the next day. So Leyzoala put the rest of the force in lockdown at a nearby hotel, forcing the cartels to attack his officers on patrol rather than at home after they got off work. Leyzoala's cops were better trained than cartel members and won most shootouts. Not long after, Leyzoala "purified" his force, firing or arresting roughly 800 dirty cops.

He then embarked on a campaign seemingly drawn from Community Policing 101, dividing the city up into sectors and instructing his sector chiefs to meet regularly with community leaders to talk about safety concerns. He also empowered his officers to go after any crime they saw, big or small -- a radical step in Mexico, where drug trafficking is generally the purview of federal police. He made allies in the state prosecutor's office, which began trying a higher percentage of murders (still only 11 percent each year, but up from 2 percent before he arrived), and the local business community, which started renovating abandoned buildings and lobbying for federal funding for youth programs and extra security downtown.