That means that both countries are growing more dependent on this relationship every day. Mexicans all across the interior follow the North Star, chasing the jobs. There are now four or five cities the size of Cleveland sitting right next door, and 25 years from now as much as 40% of the entire Mexican population may be living on the border. The region is Mexico's economic engine, a huge commercial classroom where the unskilled workers who were making gauze eye patches in 1980 now make atms and modems and the most popular Sony color TV sold in the U.S.

As for the U.S., we import not just the gizmos and gadgets but also a way of life, thanks to a shadow labor force that lets us eat out once a week because restaurants can hire dishwashers for sub-minimum wage. We depend on the maids and gardeners and carpenters and home-health-care workers whose children will probably become teachers and technicians and surgeons and Senators. If they all put down their tools tomorrow, we wouldn't be arguing about whether we are in a recession.

It's often said that the border is its own country, "amexica," neither Mexican nor American. "The border is not where the U.S. stops and Mexico begins," says Laredo mayor Betty Flores. "It's where the U.S. blends into Mexico." Both sides regard their sovereign governments as distant and dysfunctional. They are proud of their ability to take care of themselves, solve their problems faster and cheaper than any faraway bureaucrat. The Brownsville fire trucks answer sirens on the other side; in Tijuana, health clinics send shuttle buses every morning to meet people coming over for everything from dentistry to dialysis. The school district in Mission, Texas, among the state's poorest, sends its old furniture over the border to help Mexican schools that are lucky to have roofs, much less desks and chairs. El Paso is redesigning the kilns of Juarez brickmakers to cut the soot from burning old tires; the twin cities have signed more treaties than their national governments can keep track of, much less ratify. "The only way the cities in this region can make it," says Juarez mayor Gustavo Elizondo, "is to forget that a line and a river exist here."

And yet for all the frontier pioneer spirit, local leaders do draw a line: Why should the whole country benefit from the blessings of free trade, if the border region pays the price? It costs border counties $108 million a year in law enforcement and medical expenses associated with illegal crossings, money most of these poor counties can't afford, to enforce immigration policies over which they have no control. Yes, there is a shortage of truck drivers, but there is also a shortage of judges to hear all the drug and smuggling cases. Arizona ambulance companies face bankruptcy because of all the unreimbursed costs of rescuing illegals from the desert. Schools everywhere down here are poor, overcrowded and growing. Truck traffic is good for your business but bad for your health; many border cities routinely fail to meet federal air quality standards. Border agents get sick from standing on the bridges and inhaling diesel exhaust all day.

Good health care has always been scarce here, but the border boom makes it worse: a third of all tuberculosis cases in the U.S. are concentrated in the four border states. Among the hospitals in El Paso, 50% of the patients are on some kind of public assistance, mainly Medicaid. Just about the only patients paying full freight, up front, are rich Mexicans who cross over to see a specialist. "Border towns have a double burden of disease," says Russell Bennett, chief of the U.S.-Mexico Border Health Commission: "those of emerging nations, like diarrhea, as well as [First World] diseases like stress and diabetes."

The poor on both sides are united by a struggle just to survive what most Americans can barely imagine. In the rural El Paso outpost of Revolución, mothers cross into Juarez to buy methyl parathion, a pesticide so lethal it is banned in the U.S.; they sprinkle it around their shanties, and it kills the roaches and tarantulas for a year. But their children play in that dust and dirt, and when they get sick, their parents take them to Juarez doctors, who are cheaper and stay open into the night. If the children die, they are buried across the border; it costs about $150, instead of the $2,000 for an American grave.

Local officials are forever pestering the feds for help: If you don't build another bridge and put more Customs people on the ones we have, how can we solve our pollution problem, with 15-year-old cars idling in lines that stretch for miles? How can you order us to educate any child who appears on our school doorstep but not give us the money to do it? Where are we going to find enough water? The congressional Hispanic caucus wants $1 billion in spending on roads and bridges and Customs officers; El Paso state senator Eliot Shapleigh and other Texas lawmakers have called for a Marshall Plan for the border; El Paso Congressman Sylvestre Reyes wants Bush to appoint a border czar who could cut through the red tape and make things happen.

For the first time in years, maybe ever, both the U.S. and Mexico have leaders who understand this region, know that in some ways their hemisphere's economic future may depend on whether they can fix what is broken here. Bush met with Fox three times in his first 100 days, blowing away the old once-a-year tradition. Fox dreams of a day when the border is open, and his countrymen no longer flee to survive. As Fox told Ernesto Ruffo, his top aide on the region, "Put holes in the border."