ALTAR, Mexico - The police chiefs met in the dusty plaza with a federal official clutching a black bag filled with pesos: $40,000 in government pensions for the senior citizens living in the pueblos of the nearby foothills.

A convoy of seven vehicles rumbled into the plaza, the trucks squeezing between taco and T-shirt vendors who gawked at the 60 or so federal and state police officers toting assault rifles.

The crack squad had captured drug-cartel kingpins and battled gangs from Baja California to Michoacan. On this day they slipped on their ski masks to escort the police chiefs on a mission of mercy to a lost corner of Mexico.

They would be heading deep into the scrublands of the Sonoran Desert, where hundreds of cartel gunmen controlled the pueblos and ambushed intruders on hillside roads that have become blood-spattered shooting galleries.

The convoy was outmanned, outgunned and probably didn't even have the element of surprise. Cartel lookouts - they could be anybody: taxi drivers, store owners, fellow cops - had no doubt already tipped off the organized-crime groups. Cell-phone conversations were routinely intercepted.

"I'm talking here and the mafia is listening," said one commander who, like many police, residents and officials, spoke on condition of anonymity out of security concerns. "They already know we're coming."

The convoy turned past the small church and the local newspaper office, its windows blasted out, and ran every red light and stop sign leaving town.

This is Mexico's hidden drug war.

Ciudad Juarez and other violence-torn urban areas may rack up large body counts and capture headlines and presidential visits. But here in the northern part of the state of Sonora, two of Mexico's strongest drug cartels are waging a battle for scores of human and drug trafficking routes into Arizona that may be just as sinister.

One of the gangs is using a slow, bloodless strategy of patience over confrontation: It's trying to starve out its rivals.

The result is a siege of medieval proportions that has cut off a region about the size of Rhode Island from government services, and severed a lifeline to thousands of ranch hands, storekeepers and retirees. Few dare leaving on the roads, and even fewer brave going in.

"Nobody will guarantee my security," said Juan Alberto Lopez, a consultant who was supposed to drive up into the foothills for meetings with pueblo officials. "They told me they would come down to Altar," he said. "But they haven't shown up."

The war escalated this summer when gunmen from the Beltran-Leyva cartel took over the string of pueblos and ranch lands stretching 50 miles from Altar to the Arizona border. Their foes from the Sinaloa drug cartel have since surrounded them. They patrol the four main winding roads leading in and out of the hills and block almost all food and gasoline shipments.

There have been massacres and scores of kidnappings, but the war has gone largely unnoticed because of its remote location, intimidation of the media and the slow-motion tactics.

"The problem is that one gang is hiding out, very well-concealed," said a high-level Sonora state law enforcement official. "And the other group wants to get them out, to restore control over that area."

Caught in the middle are an estimated 5,000 people who every day wake up with questions: Were there any kidnappings overnight? Have the gunmen taken over another ranch? Are there any tortillas in the store?

One grandmother in Saric, grief-stricken over the kidnapping of three sons, said she tried to get help from the mayor, but he hasn't been seen in days.

She's losing hope: "Our town is dying."

Before heading out on its 40-mile journey into the foothills, the convoy took over all the pumps at a Pemex gasoline station. The officers bought sodas and chips, and stuffed them into their bag lunches; food might be scarce along the way.

The police chiefs shook hands with some of the officers. It wasn't clear whether they were greetings or wishes of good luck..

Few reporters have ventured into the area, and public officials refuse to provide much information, fearing retaliation. Since September, two mayors, a police chief and at least 11 officers have fled, joining hundreds, perhaps thousands, of residents who had also abandoned the region because of the tightening siege.

Hungry, encircled gunmen have invaded ranches to slaughter cattle. They roam pueblos in large convoys, kidnapping people and tossing their tortured bodies into the road. Many residents stay indoors when night falls, avoiding contact with the Beltran-Leyva gunmen, and stay off the roads for fear of being stopped at highway checkpoints run by the Sinaloa gang.

"We're living desperate times here. They're not letting supplies through. … We're down to basics, beans and potatoes," said one longtime female resident of Tubutama, a pueblo perched on a mesa and known for its white-washed mission church and plaza, where locals and visiting Americans on mission tours once sipped drinks and listened to bands on summer nights.

The two cartels are warring for Mexico's most valuable corridor for smuggling people into the U.S., with an infrastructure of drivers, guides, suppliers and fleabag hotels that has pumped millions of immigrants across the border. Each cartel has allied itself with local gangs with names like the Wild Boars and the Masked Ones.

In the scorching valley south of the foothills, most residents appear to have sided with the Sinaloa group, saying they at least have brought order to the messy business of smuggling drugs and people across the border.

Cartel toll takers monitor the Altar-Sasabe highway leading toward the frontier, making sure each immigrant-loaded van has paid the $100-per-immigrant protection fee. Rogue gangs that preyed on vulnerable immigrants have been chased out by the cartel, say some residents and immigrant safety groups.

Life in the valley follows a relatively secure, if hyper-vigilant, routine. When a pair of reporters walked through the town of Pitiquito a day before the convoy hit the road, a pack of teenagers and men wielding a club and a baseball bat descended on them. "Whose side are you on? What are you doing here?" one of them asked.

A middle-aged woman walking with her teenage daughter later explained that the town was controlled by a young Sinaloan crime boss greatly respected by residents. Two of his gunmen had joined hundreds that afternoon in a funeral procession for a popular musician killed in an accident. The crime boss had probably paid for the funeral, she said.

"He's the one on our side (of the war)," one woman said. "He is a generous man and protects us. Nobody is even allowed to sell drugs here. Everybody loves him here."

In the sparsely populated foothill towns known as the pueblos de arriba - the towns up above - expressing such sentiments can be lethal.

The government force began its steady ascent on the two-lane road and passed through the pueblo of Atil, where many residents avoid using telephones, believing the cartels can listen in.

One former resident, a middle-aged woman, said her son was kidnapped and killed this year, and that the family had to flee with a mattress strapped to their pickup truck. Though she's concerned for family members left behind in Atil, she won't call them.