In late 2010, after the ninth corpse or body part had been discovered on his ranch in a span of 12 months, David Lowell sat down and drafted a document that he later took to calling, with a grain of dark pride, "my map of atrocities." Lowell lives in southern Arizona, 11 miles north of Mexico, in a hinterland canyon in the middle of the busiest drug- and human-smuggling corridor in the United States. Lowell's map, "Sites of Recent Border Violence Within the Atascosa Ranch," renders the ranch boundary as a thick black line. Inside the line glow 17 red dots, each stamped with a number. Among the descriptions in the corresponding key: "Rape tree with women's underwear" (2); "Fresh human head without body" (3); "Skull" (3A); "Body found 500 yards west of Lowell home" (6); "Body found 100 yards south of Lowell home" (7); and "Patrolman Terry killed by Mexican bandits" (12).

Lowell, who is 84, has owned and run the Atascosa Ranch for more than 35 years. He is slight and chalk-pallid but possessed of a steady vigor. He handed me a copy of the map in his office before taking me on a tour of the ranch. "In the case of the human head," Lowell said as I was examining the map, "one of our cowboys came to the house holding a Safeway bag and said, 'You wanna see something interesting?' And I said, 'Sure,' and I opened the bag and inside there was a fairly fresh human head. Meat. Fresh-looking meat."

After telling the cowboy, Martin, to put the head back exactly where he'd found it, Lowell called the Santa Cruz County sheriff's office. Neither the responding deputies nor Lowell nor Martin could find a corpse. Eventually, the county medical examiner matched the head to the remains of a body recovered a mile away. The deceased was an illegal immigrant who had probably been abandoned by his guide and died of hunger or exposure. Animals had dissected his corpse.

About five years earlier, the Sinaloa Cartel, one of Mexico's most powerful drug-­trafficking organizations, had assumed control of the smuggling corridor, which runs from Nogales to Phoenix and is roughly 8,000 square miles. All of the human smugglers work for the cartel now. ("The days of the independent coyote are gone," several locals told me.) Decapitated enemies and illegal immigrants left to die are the detritus of a newly disciplined, unitary system.

The Nogales–Phoenix corridor is one of the roughest, least-accessible swaths of land along the U.S.–Mexico border. It became the Sinaloa Cartel's primary trafficking route not long after the 9/11 attacks, when "border security" was a touchstone phrase and lawmakers worried that bomb-bearing terrorists posing as illegal immigrants would exploit the country's permeable southwestern border. The Department of Homeland Security sought to move beyond mere deterrence and achieve "operational control" of the border. It assigned the Border Patrol a special "priority mission": to prevent "terrorists and terrorists' weapons, including weapons of mass destruction, from entering the United States." Legislation in 2004 and 2006 provided funding for 10,000 additional Border Patrol agents, mandated 700 miles of new fencing, and authorized the deployment of advanced surveillance systems: laser range finders, mobile ground radar, unmanned aerial vehicles, infrared cameras.

The technology and agents are in place, and much of the fence is complete, but the new tactics seem to have rerouted illicit traffic as much as decreased it. With superior fencing and detection equipment, and a recent burst of manpower, DHS has successfully shielded populated areas and shut down some major trafficking routes. Elsewhere, though, agents are scarcer, and the new fence is not continuous or uniform: Pedestrian fencing alternates with vehicle barriers and long stretches of open border. Much of the Nogales–Phoenix corridor is, in any event, so mountainous as to be unfenceable, and its isolation and immensity make it inimical to law enforcement. As DHS selectively tightened the border and drug seizures increased, the cartel moved into the corridor. The infrastructure and counter-­surveillance systems it has established there make its operations virtually ineradicable. It now ships an enormous volume of narcotics through the corridor each year, including as much as a quarter of all the marijuana that enters the United States.

Lowell occasionally sees smugglers hiking through canyons on the ranch. There are typically 10 porters and two armed guards. "They're very often a matched team, all about the same size, muscular and in good condition. Our policy is to turn off at a right angle or go back the way we'd come. I've had one or two experiences where I've thought, 'Somebody might really take a shot at us.'"

It is unlikely that a trafficker would deliberately shoot at a U.S. citizen. The more attention traffickers attract, the less efficient they become, and cartel bosses prioritize efficiency. "The Sinaloa Cartel includes some of the best entrepreneurs of all time," a senior DEA agent based in Tucson told me. "These guys know how to make billions of dollars better than anybody." They prefer their assault rifles to be used against bandits.

Confrontations between traffickers and bandits, or "rip crews," account for most of the violence here. When bandits become too disruptive, the cartel reportedly deploys teams of assassins. In 2010, a sheriff in Florence received an intelligence report from the DHS: Cartel leaders planned to send "a group of 15 very well-equipped and armed sicarios [assassins]" into the Vekol Valley, south of Phoenix. A month later, two men were killed there, in what appeared to be a sicario ambush.

We left the office, taking with us Lowell's springer spaniels, Ginger and Spook. The Sonoran monsoon had greened the canyon. Lowell's house, built of stone and 100-year-old adobe, stands above the canyon's wash, which had just received a braid of water. To the west, the canyon climbs into the foothills of the Atascosa Mountains. To the east, it widens into a valley that approaches the Santa Rita Mountains.

Lowell said that the cartel had scouts on mountaintops across the ranch. "I'm sure people will be watching us today," he said.

I said that must be disturbing.

"Oh, we kind of forget about it," he said. "But one of the dilemmas we do run into is that, fairly often, we see groups coming past our house – maybe a hundred feet away."

Until recently, Lowell and his wife, Edith, reported these sightings to the Border Patrol. "But after 2010 – 14 dead bodies, or people shot at, or people killed – we're not quite so enthusiastic about calling," he said. In the last two years, unknown assailants shot and killed an Arizona rancher named Rob Krentz and a New Mexico landowner named Larry Link. Rumors along the border hold that they were murdered in retaliation for reporting drug activity.

Edith, who had been running errands in Nogales, pulled up to the house and came over to say hello. "There was a helicopter hovering over the ridge, and two cars stopped where I drove in, and two female Border Patrol agents were out of their vehicle," she said. "So, a little action today." She told us to have a good time and walked inside.

The helicopter returned and hovered at the end of the Lowells' driveway. The Border Patrol had increased its presence in the canyon after bandits killed an agent named Brian Terry in December 2010 (number 12 on Lowell's map), but until then traffickers had used the intersection of the driveway and a county road, Avenida Beatriz, as a vehicle-staging area.

The concentration of agents has just pushed traffic into adjacent canyons, Lowell said. "The cartel is still bringing drugs down this canyon on a regular basis."

We climbed into his SUV and started down the driveway, immediately passing the ranch junkyard and hay barn. Loads of dope had been found in both places on multiple occasions. Border Patrol agents once found paraphernalia in the hay barn; a drug mule had been smoking marijuana out of a soda-can pipe. "Which was not optimal from our point of view," Lowell said, "because of the risk of fire."

One night Edith found a car parked in the middle of the driveway. Annoyed, she approached it. "To her horror, the trunk was open and two fellows were filling it with marijuana," Lowell said. "They looked at her, and she looked at them, until they'd gotten all the marijuana in the car and rode off."

We passed the Border Patrol SUV at the end of the driveway and began a steep climb up the canyon ridge. The road turned to dirt and the ridge narrowed. Ravines fell away on either side. In all directions sharp canyonlands stretched to spurred foothills and peaks. It was a landscape human beings should be moving around, not through. Sheriff's deputies sometimes had to be lowered into it by helicopter.

"One dead body was over there," Lowell said, stopping and indicating a point on a ridge. "Between there and our canyon were two others. The human head was found near where that road disappears. A second human head was found in the last year in about the same place. But it was an older head, more of a skull. We have one rape tree" – where the smugglers rape women they're guiding – "that I know about. But we have a neighbor who likes everything tidy, and she went up and collected all the women's underwear."

Two friends of Lowell's, out hiking southwest of the ranch, had found a tree with 32 pairs of women's underwear hanging from its branches.

"Some of them showed signs of having been there for a year or two," Lowell said. "It was a repeated rape site. There's some really bad people involved in this."

Later, back at the house, Edith joined us in the living room. The windows afford a view of the wash and the ridge beyond. I asked if it was hard to relax in the evenings.

"Well, we're careful to pull the curtains," she said.

"And we automatically lock all the doors," Lowell said. "We have a sophisticated alarm system with motion-sensing lights and three sirens, and we have guns."

Someone had tried to break into their bedroom once, but the dogs had scared him off. Six men had snuck up to the house and demanded money and food from Lowell's secretary, but she'd run them off with the dogs, too. A few years earlier, they had hosted an event for a state representative who was looking into the trafficking situation. Edith told the representative that drug carriers walked right by the house, and pointed out a window. "And right then I looked out, and here came a group of drug mules," Lowell told me. "In our yard! Fifty feet from the house. And they didn't pay any attention to us. They just hiked by with their packs."

One night, he said, Ginger started growling, and when he let her out she stopped on the porch and pointed. "I pretended that her head was a rifle sight, and I got behind her and sighted between her ears and over her nose. Soon two fellows walked through a gap in the bushes. Maybe 150 feet away."

"If Ginger starts barking loudly, we know there's someone around," Edith said. "Once in a while we hear noises getting closer, and our lights go on, and we get alarmed enough to get up and look out. Otherwise, we have peaceful evenings."Lowell's neighbor Jim Chilton owns a 50,000-acre ranch that shares a five-mile border with Mexico. Hundreds of smuggling trails cross it. Guided by scouts and aided by a paucity of roads and law-­enforcement patrols, the smugglers move easily through the terrain.

When I showed up at his house, Chilton opened a small barred window set into his heavy front door and scrutinized me for a moment, then invited me in for cowboy coffee in his kitchen with his wife, Sue. Chilton is 72, Sue, 69. They are relaxed and of modest stature. Sue, a naturalist, said that at 3,500 feet, mesquite grassland gives way to oak grassland and that the region has a bimodal weather pattern (convective and frontal). With a cackle and sidelong glance, Chilton said, "I just read cowboy things."

"Oh, don't believe him," said Sue. "He has a great, great interest in history."

Chilton, whose family has been ranching in Arizona for five generations, has a large library where he likes to sit and read Bancroft's Works or 19th-century magazine articles with titles like "My Adventures in Zuñi."

Chilton brought a Ruger .44 pistol and a Ruger .223 rifle with us out to his pickup, and we drove south into a landscape that lay furrowed or broken between mountain ranges. Ranchers here lease most of their land from the state or federal government, and their permits restrict material improvements. The majority of the corridor runs through national forests or monuments, or semiautonomous, undeveloped Native American reservations. Law-enforcement officers consequently have very few roads to use and no operating bases – 96 percent of Chilton's ranch is publicly owned. Its scarce roads are gashed and serpentine.

"How on Earth," Chilton asked, "would a border patrolman ever see anyone coming through this country?" He slowed above a notably green arroyo called Yellowjacket Pasture. "The Border Patrol rarely get out of their vehicles – they patrol the roads. Now imagine you got people down there in Yellowjacket Wash. Look down there. Look at all the hiding spots."

The wash had produced thick stands of mesquite trees, and an opaque profusion of ocotillo and guajilla and creosote and prickly pear cactus.

As we continued south, Chilton began pointing out trail after trail coming down through the hills: little switchback scars, bare tracks descending arroyos. "There are literally hundreds," he said. "There's no way law enforcement can cover all these trails." Smugglers could cross the border at night without resistance, Chilton said, climb the mountains, come down these trails, and disappear.

Like Lowell, Chilton told me that scouts have stationed themselves on mountaintops all across his ranch. According to the DEA, the Sinaloa Cartel employs between 200 and 300 surveillance teams along the length of the corridor. Drug loads can be passed from one team to another, all the way north. Scouts have night-vision goggles, infrared telescopes, and military-grade two-way radios with rolling encryption. Land-mobile radio repeaters boost the strength of outgoing transmissions, so no signal degradation occurs as the radio waves flow north. Portable solar panels power the devices. The cartel can reprovision its surveillance teams over a period of months a hundred miles into the United States.

Individual scouts can be chased from their locations, but only temporarily, and they usually succeed in remaining invisible. The mountains are full of caves – "spider holes," Border Patrol and DEA agents call them – and when scouts can't find caves they remain deep in brush, beneath rock overhangs or under camouflage tarps. When scouts are spotted and flushed, they're rarely caught. If you see them, they see you, and they have strict orders, Chilton said, to drop everything and run. The cartel doesn't care about losing equipment; it worries about giving up information. "Chasing a scout," one Border Patrol agent told me, "is like chasing a unicorn."

Shortly afterward, we saw a Border Patrol SUV parked on the side of the road. "He's there to prevent vehicle traffic," Chilton said. "He'll just sit there." In the Border Patrol's Tucson sector, which encompasses the corridor, the agency has at most one agent for every 21 square miles of territory. We would spend seven hours traversing the ranch that day and not see another patrol unit.

The farther south we drove, the rougher the terrain became. We entered the Bartolo Mountains. Halfway down a narrow canyon, Chilton pulled over. The other side of the canyon, he said, was Mexico. He grabbed his rifle, and we got out and climbed down to a small plateau. A cattle fence, making acute angles as it traveled through the mountains, marked the border. The fence was so low and insubstantial, it disappeared as soon as you shifted your gaze. We were far beyond the scope of any patrol, Chilton said. He doubted the cartel even bothered to station scouts here.I returned to the border the following day with Deputy Omar Rodriguez, a member of the Santa Cruz County sheriff's office. A good portion of the southern half of the smuggling corridor runs through Santa Cruz County. Our patrol was part of Operation Stonegarden, an anti-trafficking initiative whose objective was to get more "eyes on the ground." A guarded, thoughtful, crew-cut realist, Rodriguez had been a deputy since 2006, and there were parts of the county he still hadn't seen. In addition to the detection problem so evident during my ride with Chilton, law enforcement also faced an apprehension problem. To maximize coverage, Rodriguez said, deputies went out alone – but couldn't apprehend armed traffickers without backup. And the traffickers usually got away before backup arrived.

The patrol with Rodriguez would serve as a kind of inventory of the advantages enjoyed by the traffickers: cartel engineers who devised methods for circumventing physical barriers; U.S. citizens on the traffickers' payroll who provided logistical support; scouts who meticulously documented the habits of U.S. law enforcement. From Nogales, the county seat, we drove northeast into the Coronado National Forest, passing through the Patagonia Mountains, some of whose peaks approach 7,000 feet. Several signs along the road read smuggling and illegal immigration may be encountered in this area. Now and again we saw trailers and houses. I asked Rodriguez whether he'd be comfortable living there, or even hiking. "Me, personally?" he said. "No."

We turned south, descended steadily, and came upon a handful of singlewide trailers and tar-shingled bungalows. Ahead of us, an arroyo widened into a weedy field, and the road ran under a steel gate. "That's Mexico," Rodriguez said. A derelict sign on the gate faintly read '1975' and 'Lochiel Port of Entry.' Low, anti-vehicle Normandy barriers, recently installed, ran along the border. Rodriguez said that traffickers use flatbed tow trucks to drop dope-filled vehicles over the barriers. They drive over higher vehicle barriers on portable, custom-built metal bridges. When I asked him how much of the Border Patrol's most formidable fencing – deeply anchored steel posts with concrete cores, 30 feet high – protects Santa Cruz County, the fullness and duration of his laugh surprised me. "Not much," he said. Later I got the exact figure: 2.8 miles.

Traffickers know a lot about law enforcement on this side of the border. They see almost everything. They hire U.S. citizens to collect intelligence. A person living on a fixed income in a mobile home in the Coronado National Forest might accept a stack of bills from a stranger if all they have to do in exchange is call a number on a prepaid cellphone when a sheriff's deputy drives by or a Border Patrol technician installs a sensor. Traffickers hire locals to obtain police reports and press releases after major seizures and debrief drug mules forced to drop their loads. They will know which agencies were involved, which kinds of vehicles were used, whether air support was available.

Cartel surveillance teams generally know how long it will take a law-enforcement unit to get from one point to another – they measure response times. They are familiar with the protocols of Border Patrol shift changes. They know that there are fewer agents in the field on weekends. They have mapped everything – all the forest lanes wavering away from the Lochiel gate, for example, as well as the dead-end spur roads. They know whether the Border Patrol has been using trackers in an area and how much lead time a group will need to outpace them.

If a vehicle crossing the border at Lochiel trips a sensor or is otherwise detected and law enforcement responds, scouts direct it onto a spur road, where its driver covers it with brush and a camouflage tarp. (Scouts may also note the potential presence of a new sensor.) Already provisioned for this eventuality, drivers will wait for minutes or hours or days, until the roads are clear.

Traffickers use decoy groups to walk across the border at known sensor locations. Or they may employ banzais, who simultaneously scale border fences and scatter, vacuuming up manpower. Jim Chilton told me that 12 men with assault rifles once marched across the border and straight at a National Guard surveillance post. The men paused while the alarm rippled through the system and then crossed back. As Border Patrol units and tactical teams and sheriff's deputies and helicopters descended on the post, smugglers crossed en masse for miles on either side.

From Lochiel, we headed to the Santa Cruz River, which runs into Mexico about 10 miles east of Nogales. Drug mules often cross the border and hide in the brush along its banks until scouts signal them to continue. We pulled over a few miles north of the border and began walking along the river's south bank until we came to a camp. Amid a jetsam of empty water bottles and bleached shreds of cloth and bits of clothing stood improvised shelters: a lean-to of tarp and driftwood stakes, a canopy of tarp pulled through a lattice of low branches, a roof of viney undergrowth and cardboard on legs of salvaged PVC pipe. We found another camp close by, and then another, and another.

Rodriguez said that traffickers pay locals to resupply people in the camps. He had stopped river-bound cars full of pizza, roasted chicken, and soda, all purchased in bulk from Walmart. In one scenario, a driver picks up provisions at Walmart while three sentinels with cellphones and binoculars station themselves between the store and a drop-off point along the river. If no law enforcement is present, the sentinels authorize the drop and the driver deposits the supplies and leaves. None of these U.S. citizens interacts with cartel traffickers or visibly break the law, and none have information beyond a simple set of instructions for performing a discrete task.The corridor's smuggling routes complete their convergence in Pinal County, which extends south from Phoenix. For the past five years, the county has seen unprecedented levels of drug trafficking and violence.

"It's hard to tell how much violence there actually is because there's so much shit that goes on out here we don't see," Lt. Matthew Thomas, commander of the Pinal Regional SWAT team, told me. The county's 220 deputies patrol 5,500 square miles. Thomas conducts some of the most aggressive anti-narcotics operations in the smuggling corridor.

I met Thomas one morning before dawn at a sheriff's office substation in the dilapidated desert town of Arizona City. We climbed into his unmarked silver Chevy Tahoe and turned onto Sunland Gin Road, a thoroughfare sometimes used by traffickers. He wore a long-sleeve SWAT T-shirt, desert army pants, and beige combat boots. He had a tactical knife in a scabbard strapped to one leg.

Earlier that morning, Thomas told me, blacked-out SWAT vehicles had dropped off eight deputies near surveillance positions in two nearby mountain ranges that smugglers use to move north through the county.

"If we keep relatively current on the activity of smuggling routes, we might catch somebody," Thomas said. "The smugglers switch routes, but they might keep a successful one going a little longer than they should."

Because the cartel's own surveillance coverage is so advanced and comprehensive, Thomas can never be sure whether his men will make it to their positions unseen.

"The bosses know when the dope is moving, and they'll start asking, 'What's out there?'" Thomas said. "They've mapped the area and all their scout locations. If it's hot they'll say, 'Shut it down,' and the smugglers will run their load into a wash, cover it with camo and brush, and wait. When they do that, it's very hard to find them. They're very good at hiding. And they'll wait as long as days."

The cartel's resupply vehicles sometimes spend all night provisioning scout positions, making the rounds from one peak to the next. Occasionally they drop off prostitutes for a day or two. Two weeks earlier, Thomas had impounded a car containing 200 pounds of dope and copious supplies for scouts. He pulled out his cellphone and showed me pictures of the supplies: packets of socks and underwear; Levi's jeans with the tags still on them; cans of beans and bags of tortillas; cases of Sprite, Coke, and Gatorade; bottles of ­tequila and beer; a carton of Marlboros; a Glock 9mm pistol; a two-way radio charger and two phone chargers, all wired to portable solar panels; Remington and American Eagle ammunition (9mm, .38 super, .22); and an all-weather Puma smartphone with embedded solar panels. Still in its case, the phone had just been released in Europe. It wasn't available yet in the United States.

Traffickers make their own roads, running one three-quarter-ton pickup right behind another, crushing vegetation and replacing tires and vehicles as needed. The drivers navigate a landscape of washes, mesquite thickets, irrigation berms, and foothill canyons.

"I'll tell you what, their drivers amaze me," Thomas said. If the SWAT team can't overtake smugglers, they'll try to push them toward Border Patrol tactical teams. "But at night we have to have an air asset in front of us with infrared and night vision, because if we're pushing them we need to know where to set up choke points."

SWAT operations yield apprehensions and seizures, but to be effective they mostly rely on the criminals making mistakes. Thomas and his men have to hope that smugglers will overuse routes, that cartel scouts won't discover SWAT surveillance positions, that traffickers' evasion techniques will fail, that an air asset will be available.

"There's no misconception that we're ever ahead on any of this stuff," he said. "They've got more time and more money and more manpower."

The sky lightened as we drove through cotton fields and mesquite flats. We entered a construction zone, slowed to a crawl, and passed a dented pickup in the opposite lane. Its occupants glanced at us, then glanced again.

"It's highly likely they're a lot more than farmworkers," Thomas said. From a distance, the Tahoe looked like any other SUV, but up close you noticed its antennas and the maximum tint of its windows. "If we think we've been made by scouts, we try to act natural. Let them think it's just some dumb cop on a regular patrol."

One by one, Thomas' men radioed in to say that they'd seen no activity that night and dawn had revealed no traces of movement along the routes. Another SWAT team member radioed in. He'd picked up a drug mule standing by the side of the road. The mule had delivered his load sometime earlier that night – he had the telltale backpack ruts on his shoulders – and he knew that he could only be charged with illegal entry. He would likely be processed and bused home, which he preferred to walking. The dope in his pack might sell for $70,000, nearly twice the salary of a first-year deputy.

Thomas agreed to show me the notorious Vekol Valley, the last leg of an alternate smuggling route running up the western edge of Pinal County. We drove west on Interstate 8 and entered the Sonoran Desert National Monument on Vekol Valley Road. Scouts often sit in the brush here, guiding smugglers north through the desert to load-out spots along the highway. We turned onto a Bureau of Land Management road, which was hardly graded and not easily distinguishable from the land itself, and stopped at a hollow below a ridge joining two low hills. Thomas told me a trafficker had recently killed a rip crew member here.

We climbed one of the hills. The Vekol Valley opened out below us. In some di­rections, you could see for 20 miles. Law-enforcement vehicles would be easy for an unaided eye to pick out. With night-vision goggles, you could spot a blacked-out SUV. Disciplined scouts with high-powered binoculars and infrared telescopes would see everything that moved in the valley.

We drove back to I-8 and pulled over. "There's so much traffic that any place that's a natural hiding spot along the highway, you can go in there and it's pretty much guaranteed to be a load-out spot," Thomas said. We climbed down an embankment and into a brushy arroyo. In the arroyo were backpacks, blankets, sweatshirts, empty bottles of the rehydrating sports drink Electrolit, disposable razors, burlap sacks for marijuana bundles, four pairs of "carpet shoes" (or "sneaky feet") to confuse trackers, and the same sort of improvised shelters I'd seen along the banks of the Santa Cruz River, 150 miles to the south.

On the way back to Arizona City, Thomas said, "Realistically, we know we're not stopping the flow. When you debrief people on the cartel side, when you get all the drugs and people you seize and you know how much actually makes it north, it's not much. On a good day we might get 20 percent. Normally we're probably getting five to 10 percent. And I would say that's everyone: Border Patrol, Immigration and Customs Enforcement, the other task forces." A few years back, he said, Pinal SWAT seized an unprecedented 10,000 pounds of dope in a month. Later, one of Thomas' informants told him that the cartel was moving 15,000 pounds through the county every week.Some law-enforcement officials, and many conservative politicians, advocate militarizing the border. "We need 6,000 armed soldiers on our border to protect America," Matthew Thomas' boss, Sheriff Paul Babeu, recently said. "Commit the military to this border," former Colorado congressman and 2008 Republican presidential candidate Tom Tancredo has demanded. "We have a war. We are facing a military on the other side." Texas Governor Rick Perry has said that he considers the border "a war zone as dangerous as Iraq."

It's instructive to imagine what militarizing the border would actually require. Rich Stana, the former director of homeland security for the U.S. Government Accountability Office, has said that a zero-incursion barrier would be "something akin to the inner German border during the Cold War, where very few, if any, could penetrate it without fear of losing one's life." A militarization project would entail blasting and grading a security zone from the Pacific Ocean to the Gulf of Mexico, erecting 1,900 miles of double or triple fence along its length, and deploying many thousands of soldiers to man guard towers and patrol the zone. (South Korea, by way of comparison, has more than 250,000 troops stationed along its 150-mile border with North Korea.) The soldiers would be authorized to shoot and kill Mexican citizens. In the meantime, demand for drugs in this country would remain stable. Our coasts and airspace and wide-open 5,500-mile-long border with Canada would welcome pioneering importers.

Sheriff Tony Estrada, Deputy Omar Rodri­guez's boss, does not take militarization seriously. "I always say, 'The border is more secure than it's ever been; it's just not sealed,'" he told me. "It cannot be sealed. You have tourism, you have international commerce. If you want zero everything – mission impossible." He measures border security by the local crime rate, which is low in his county. He understands the suffering of ranchers like David Lowell and Jim Chilton, but his priority is the overall safety of the county.

Unless we're willing to establish a DMZ along the southwestern border, this is the only rational perspective available. The ­Department of Homeland Security has already adopted it. "The specific theory of action [is] to push people out of easy urban places to cross the border and get into the transportation network," the former commissioner of U.S. Customs and Border Protection, Alan Bersin, said last summer. DHS deters and apprehends wherever it can, and it forces traffickers around populous areas. That strategy, of course, will always be problematic for the people living in the traffic's redirected currents, who justifiably feel as though they have accidentally moved out of their own country.Consider in that context the residents of the Silverbell Estates, arguably the most cartel-oppressed subdivision in the United States. An exurban isolate in the open desert of southern Pinal County, it is too remote for timely police response and is by design diffuse and unassuming. The streets, named for Greek gods and the signs of the Zodiac, are unlighted; the landscaping is muted; the faux-adobe homes sit on multi-acre lots. It is also almost perfectly centered in the corridor.

Not long ago, a resident of the Estates was surveying the desert landscape from his rooftop deck when he noticed a form on nearby Wildcat Peak. He examined it through binoculars he kept on the roof for viewing wildlife. A cartel scout was standing outside of a cave.

I heard this story at the home of a retiree named Pat Murphree. We were sitting at his dining room table, drinking iced tea with his wife, Pennee, and their neighbors, Jay Stewart and Sam Schreiner. At one end of the table a picture window framed Wildcat Peak. A telescope on a tripod pointed at the scout's cave, but you could see it clearly with a glance.

People here first became aware of the trafficking in 2007, when the sound of three-quarter-ton pickups bludgeoning their way through the desert began waking them up in the middle of the night. Running without lights, the trucks tore up the two-mile fence encircling the Estates.

"We repaired, what, 11 breaks in the fence about two years ago?" Murphree asked.

"About a quarter mile of fence, wasn't it?" Stewart said.

"And about a year ago, 14 breaks were repaired, and there's probably five or six of them out there now," Murphree said.

Residents called the Border Patrol and the sheriff's department, but the trucks were usually gone before they got there, and the traffic followed no pattern.

Stewart, a US Airways pilot, flies a single-prop plane recreationally, taxiing it out of his hybrid hangar-garage and onto a short unpaved runway. From the air he started spotting caches of goods in the desert. There were also pickups and SUVs camouflaged for future use or totaled and abandoned, jugs of gasoline, spare tires with mounted rims, cases of bottled water. He once landed near a Humvee, fully loaded with features, a Coach purse on the front seat.

"That was a guy's dream vehicle. He'd worked his butt off, owns a framing company, and here these SOBs steal it," Stewart said.

Dozens and dozens of vehicles were appearing in the desert, and the numbers haven't declined. "There's so many vehicles I've quit counting," Stewart said. He has found abandoned smugglers' vehicles at the end of his driveway and groups of illegal immigrants at the end of his runway. One day he took off, looked down, and saw 40 people.

"But that's what it's like living out here," he said. "You never know what's coming."

When he sees a group, he contacts the Border Patrol, sometimes staying up and "orbiting" the illegal immigrants, holding them in place until the agents arrive.

"So you've been an air asset of the Border Patrol?" I asked.

"More than once," he said. "But I moved down here to have fun, not to fight the drug cartel."

After the scout sighting, Pinal SWAT stormed and cleared Wildcat Peak, and Stewart and Murphree hiked to the cave and began cleaning it up.

"The trash, you can't imagine," Stewart said. "They'd been going up there at least six months, and you couldn't see it – they had it all covered with camo tarp."

"I think we took out 19 bags of trash," Murphree said. "That stuff stunk so bad."

"I was standing knee-deep in that rubbish pile," Stewart said. "It was all cans of menudo, and full of scorpions and black widows, and it was gag-a-maggot. It was bad."

There are also the casualties of human trafficking.

"They cram the pickup beds with people standing up and put a rope around them," Schreiner said. "And the sway of the bodies can bend the sides of the bed about 45 degrees. They lie on the cab roof, they lie on the hood, they take all the seats out."

About four years ago, just before dawn, in front of the big white stucco pillars of the subdivision's entrance, a pickup carrying 35 illegal immigrants took a turn too fast.

"I had to dodge dead bodies going to work that morning," Stewart said.

Murphree remembers seeing 26 people lying on the ground. Four of them, he could tell, were dead. Ten eventually died. Six rescue helicopters evacuated the dead and wounded. The helicopters had to queue up in the air. The cleanup took seven hours.

Early that evening, Murphree took me to visit Susan and Nathan Cary, who live at the southern edge of the Estates and are most exposed to the traffic. The Carys are children's advocates for an international organization called Compassion in Jesus' Name, whose motto is "Releasing Children From Poverty."

In their living room, they told me that they have always tried to feed and shelter illegal immigrants abandoned by their guides. It's become frightening, though. They have suffered break-ins, and they don't know whether the perpetrators are desperate immigrants, armed drug mules, or scouts. Of a man who showed up one winter, Nathan said, "He was cold, he was cold, and I thought, 'What am I going to do with him, this poor soul.' But you don't know who they are, who they're with."

One night, Nathan said, someone killed their dog. The animal's senses had shielded the property for many years. The killer had stabbed the dog in the eye with a fine-pointed object or shot it in the eye with an air gun, Nathan couldn't tell. He recognized the pointlessness of reporting the incident, but he felt that he should somehow mark the death of a loved creature.

"I don't feel safe walking here anymore," Susan said. "When we moved here, I walked all the time."

"I walk in the morning, but I take the dog, and I take a firearm," Nathan said.

They recently fenced in the property, which has helped hold down the number of people who come to the house unbidden. They keep the gate closed, but not locked.

"I'm not a hateful man, and if people want to come in because they need to, then they can," Nathan said. "The closed gate says, 'We're not expecting you, approach respectfully.'" Although they offer strangers the shelter of their covered front porch and food and water, they also call the Border Patrol. They will allow the house to be used as a comfort station, but not a refuge.

"When you're alone, it's unsettling, but when we're together we work as a team," ­Susan said. "He speaks more Spanish than I do, and I stay in here with the phone and the gun locked and loaded."

"So if I get knocked off, she'll make the call and have the gun ready," Nathan said.

When I left, he came out with me to open the gate and make sure I had my bearings. He held a rifle in one hand and a law-­enforcement flashlight in the other. He kept the rifle barrel down but held the flashlight just above it like a scope – ready, should the worst happen, to sight and shoot a figure coming out of the night.