Border-patrol agent Nate Lagasse is sitting quietly in his Toyota Land Cruiser about three miles west of a small Arizona town on the Mexican border, following a group of 12 immigrants through his night-vision goggles. He radios directions to three colleagues, who are out in the mesquite on foot and closing in on the aliens. "They don't even know we are here yet," whispers Lagasse, who has turned off his headlights and allowed his truck to roll to a halt without hitting the brakes. "It's just like hunting."

Something alerts the aliens, and they hit the dirt, probably at the order of the coyote, or guide, they have paid to get them across. But Lagasse has marked their location and talks his three agents in on top of them. After a few minutes, a voice comes over his radio: "We have them now." The immigrants make no attempt to escape. The sight of a few agents in uniform is often enough to pacify a large group; some agents have singlehandedly detained 100 people at once.

Between 6 million and 12 million illegal aliens live in the U.S., the majority are from Mexico and most move through Arizona. It draws more than a third of the illegals, including 14 men who died of dehydration near Yuma after the temperature hit 115F degrees two weeks ago. But the busiest place in the state is the tiny border town of Naco, a place so anonymous that its name derives from the last two letters of Arizona and Mexico. Naco (pop. 800) is little more than a bar, a school, a couple of streets and 220 border-patrol agents. Across the line in Mexico is a town with the same name, 10 times the population and all the makings of a first-class staging area — guest houses, grocery stores and an army of local guides, or coyotes, to show the way.

As soon as the sun goes down, hundreds of men, women and children, armed with water bottles, toothbrushes, toilet paper and perhaps phone numbers in Phoenix, or Denver or Los Angeles, come walking, running and crawling north across the border. Each night border-patrol agents round up roughly 500 and next morning return them to Mexico, only to have them start all over again the following evening. It's a never-ending drill, often with life-and-death stakes. The border patrol says 383 people died last year attempting to cross the border from Mexico. "Is this problem solvable?" asks Victor Manjarrez, 37, top agent in the Naco station. "I think we in the border patrol are getting better at what we are doing. But with a Third World economy to the south and a First World economic power to the north, you will always have this problem."

THE GAME
Every night a busy industry gears up to test the weak points all along the border's 1,952 miles. In Tijuana smugglers cram three people into a car trunk and a fourth behind a dashboard, then drive through the customs checkpoint, hoping nobody suspects anything. In Calexico aliens float down a stream choked with toxic chemicals and sewage, betting the border patrol won't jump in to pull them out. In nearby Nogales smugglers tunnel 6 ft. under the border and funnel people through.

Around Naco everyone moves on foot. It's an elaborate game. The smugglers send out small decoy groups at dusk to confuse the agents and use metal detectors to locate the underground sensors buried at intervals along the border. The feds have erected stadium lights and video cameras at three-mile intervals down the line, enabling spotters to see anyone who crosses. Then the hard part begins. Agents must chase down every group before it reaches a road or a rendezvous point on the U.S. side, where aliens can be stuffed into trucks or cars and quickly moved north. If a group hangs together, it can be easy to find; if it "goes quail" — scatters in all directions — agents on the ground have a slimmer chance of catching everyone. But a group that splits up runs the risk of perishing in the desert.

As usual, Lagasse's radio has been crackling almost nonstop since the sun went down. An operator back in town is monitoring the sensors and reading the results over the air: "924, two hits, 956, 12 hits..." The sensors are placed under trails, arroyos and washes known to be used by smugglers. When a foot falls nearby, alerts go off at the sophisticated command-and-control center. The news is relayed to agents crisscrossing borderlands in dusty SUVs. Tonight a group of 25 aliens has been detected on South Hereford Road down by the San Pedro River. Lagasse hears of a group of 18 walking across a private ranch that has set the dogs barking; he moves eastward to cut it off. "It will be like this all night," he says, jumping out of his truck and climbing over a barbed-wire fence. It is heavy going. The ground is uneven, the bushes are thorny and there are barbed-wire fences everywhere. The trails are faint and hard to follow. On the Mexican side, trailheads are often marked with an article of women's underwear hanging on a bush or a tree. On the U.S. side, smugglers use lights, the outlines of mountains or a line of high-voltage transmission wires as landmarks. In summer, rattlers come out at night to lie on warm stones. In winter, temperatures drop below freezing. The group of 18 has disappeared into a clump of trees, but after 20 minutes, Lagasse picks it up with his night-vision goggles, heading toward the road. He and several other agents round up 12 members of the group. Six others manage to slip away in the dark.

Apprehensions have been falling in the past six months after a decade of near constant increases, partly because the feds have nailed down the big crossing points and forced more people out into the desert. But it is possible that word has gone back to Mexico by telephone that jobs in restaurants, construction, office cleaning and landscaping are not as plentiful because of the sluggish economy. Either way, there's no slowdown in Naco. From October 2000 to March 2001, Manjarrez's men turned back 56,819 aliens — up 11% from the same period a year ago, even as overall border arrests dropped 24% in that time period.