No cops came looking for him. The store owner didn’t call. No sweat. A few weeks later, the men left another bundle of cash, and Rodriguez went back to Ammo Depot and bought an AK-47, which he likewise handed off. Still no questions. A week later, the men dropped off about $10,000, and Rodriguez bought nine AR-15s. It turns out the men were driving the guns to Reynosa, Mexico, where Cárdenas Guillén and the Sinaloans were waging a fierce battle for control of the plaza. Rodriguez earned $50 a gun. Eventually, an A.T.F. agent who visited Ammo Depot noticed Rodriguez’s name coming up repeatedly in the store’s sales records. After months of tracking him, in November 2003 agents arrested Rodriguez, who by then had bought more than 150 guns. He is now serving a 70-month sentence in federal prison. The men who paid him were never caught, and only five of the guns Rodriguez bought were recovered. One was connected to the shooting of a local police officer in Reynosa. I ask the agent why Ammo Depot didn’t alert the A.T.F.; after all, Rodriguez was paying cash for dozens of weapons popular with drug traffickers. The agent’s reply: “As long as he passes the background check, it’s a completely legal sale.”

Nuevo Laredo, in Tamaulipas, has benefited as much as any city in Mexico from Nafta and cross-border trade. It has also suffered inordinately from the drug wars. The sign above the bridge connecting it to Laredo, Texas, reads GATEWAY TO THE AMERICAS, a slogan with some claim to truth. This is the busiest noncoastal commercial port of entry in either country and thus one of the most desirable drug-trafficking routes in the world. The town is hot and dusty, jammed with steamy restaurants, little banks, open-air markets, sputtering taxis, and stray dogs. It’s rimmed by abject slums that serve as stark reminders that Mexico, despite having the world’s 13th-largest economy, still struggles to take care of many of its citizens.

Beginning in the mid-1990s, Nuevo Laredo came under tighter and tighter control of the Gulf cartel. Around 2002, the cartel’s boss, Cárdenas Guillén, began recruiting defectors from a Mexican special-forces unit that, according to law-enforcement sources, had been trained in antinarcotics operations at U.S. Army bases. The defectors brought their nickname—Los Zetas—to the streets and soon developed a fearsome reputation, wiping out Cárdenas Guillén’s competition in the border plazas to the west. Guzmán, who’d long coveted the Laredo corridor, sent his own troops, including the Beltrán Leyva brothers, into the fray. Soon, northern Mexico, Nuevo Laredo in particular, was a battleground. In 2005, Nuevo Laredo’s new police chief was executed just hours after being sworn in. Vicente Fox, then Mexico’s president, sent in federal police; local police who were loyal to the Zetas shot at them. Gunfights broke out daily. In July 2005 the U.S. closed its consulate there.

Over time, as Calderón has weakened the Gulf cartel, the violence in the town has abated. Few Nuevo Laredans doubt, however, that the Zetas still exert a strong influence. When I visit in February, fear and suspicion are palpable. Soldiers and light tanks are stationed at the bridges. In a city where many cops have been fired for corruption, some still on the beat have been disarmed; they walk the streets with empty holsters. Lookouts known as halcones, or falcons, are presumed to be everywhere. While I am interviewing a State Department official in a restaurant, a shoeshine boy ducks in, looks at us, and ducks back out. “You can be sure that kid went and told somebody, who went and told somebody, who called a Zeta, and they know we’re here,” the official tells me. “They follow all of us.”

On a hot evening at the Cadillac Bar, I talk to a local newspaper reporter and Nuevo Laredo native whom I will call Ana. A few years ago, the Cadillac, a city institution, would have been packed with tourists, but on this night we are the only patrons. When the violence began, media flocked to the city, but then journalists became the objects of attacks. In 2005, a radio correspondent was shot outside her office. Gunmen sprayed the newsroom of El Mañana with AK-47 fire. Since then, Ana says, the Zetas have put many of her colleagues on the payroll to shut them up. “I don’t feel safe,” she says.