One hundred and seventy miles southwest of Tucson, hard by the Mexico line, stands a weathered mountain range called the Cabeza Prieta.  It is a place of weird landforms and scarce but formidable vegetation, a graduate school for desert rats that only the best prepared dares enter.  The geography of the place says, Stay away.  To emphasize the point, the sky rains metal as military aircraft drop payloads of bombs and spray rivers of bullets onto the vast proving ground in which the Cabeza Prieta stands.

Yet, even here, a sea of trampled backpacks, electrolyte-drink bottles, underpants, toothbrushes, and cigarette butts stretches south toward the international border, lapping at the mountains’ dry flank.  A small city’s worth of shoeprints punctuates every wash and pass: cowboy boots, tennis shoes, huarache sandals, even high heels.  They are all pointed north, leaving a broad avenue through a desert that, absent the occasional bomb crater, was, until recently, as pristine as any in North America, so much so that environmental activists in Arizona long pressed for it to be given national-monument status, with its attendant protections.  That will not happen, not while the Cabeza Prieta is a war zone, not while the sea rises and rises.

It is the same in San Diego, in Marfa, in Laredo, in Columbus.  The southern frontier of the United States, from the Pacific to the Gulf of Mexico, is a battleground, its territory contested by a vast army of would-be immigrants facing a much smaller, but still numerous, host of soldiers, police officers, and other defenders.  To call the land a battleground is by no means metaphorical, as it once would have been, for ever-larger portions of the border are being fortified and armored.  The flanks of the Ajo Mountains, not far from the Cabeza Prieta by southwestern standards, bear concrete and steel chevaux-de-frise that would not be out of place on the Normandy beaches in 1944, while the steep canyons of the Coastal Range north of Tecate, Mexico, bear rank after rank of concertina wire.

Where the fortifications are not so sturdy, the sea has broken through.  The case of the Buenos Aires National Wildlife Refuge, in south-central Arizona, is representative.  Comprising 118,000 acres of high-desert grassland, the refuge harbors several rare plant types such as the Pima pineapple cactus, along with many species of animals, including pronghorn antelope, bighorn sheep, masked bobwhite, and other creatures not often seen elsewhere.  Yet the refuge’s officers have few resources to protect these animals and plants; instead, a full third of its annual budget of about $1.5 million goes to law enforcement, while much of the rest is eaten up by such mundane details as removing trash—500 tons of it in 2005—and carting away abandoned vehicles, dozens of which lie scattered across a no man’s land of dry washes and sandy hills.  Add to them the foot traffic of as many as 3,000 alambristas, or “wire-hoppers,” per day and an endless stream of Border Patrol vehicles, ranging from all-terrain buggies to massive Humvees, and the land, once far from the center of anything in particular, once pristine, too, looks as if it has been chewed up by a wood chipper and spat out along the flanks of the rocky Baboquivari Range.

Three thousand may be a low estimate.  Indeed, no one knows how many men, women, and children cross the international frontier illegally each day.  However it is measured, the number is massive.  We can extrapolate to some extent from federal statistics, which reveal that, in 2005, Arizona’s two Border Patrol sectors apprehended 577,500 illegal aliens, nearly half of the nationwide total.  If a million are being caught each year, then it stands to reason that many more are slipping through, eventually making their way to Phoenix, Los Angeles, San Antonio—and, from there, to Virginia, North Dakota, Vermont, wherever there is work to be found.  Indeed, according to some estimates, there are now 11 million illegal immigrants in the United States, most Hispanic, most concentrated along the southern border—but with perhaps a million apiece in North Carolina, Illinois, and New York.

Why should so many people cross over such difficult terrain each year?  Never mind the motivations—and never mind the value judgments—about why they come in the first place.  It has to do with aspiration, with the have-nots wanting to live where the haves are.  It has to do with dreams.  It has to do at times with repression, at times with greed.  The chief reason that illegal crossers enter the United States through seemingly forbidding yet environmentally sensitive portals is that it is a matter of federal policy that they do so.  In the 1990’s, the Border Patrol seems to have determined that immigrant traffic through the normal ports of entry—that is, places such as Nogales, El Paso, and National City—was too vast and visible for comfort; it made for bad press when TV crews showed up and had to wait no time at all to film a wave of alambristas hopping walls and fording rivers to enter the United States, usually uncontested.  The Border Patrol therefore massed agents in those cities, driving would-be migrants out into the remote, unpatrolled desert along the length of the border.  Gone are the days of cross-border tunnels from one urban house to another; gone are the days of convenient television footage.  Now, the bulk of seizures of contraband and arrests of illegal entrants takes place in the wild country, far from the restaurants, bars, and other amenities that, uncharitable though it may be to say, make a frontier law-enforcement officer’s life a little more bearable.

Whether morale boosting and staff retention—or mere convenience—underlay the federal government’s decision is arguable, but the effects have been marked.  One is the destruction of great tracts of land, public and private, along the international frontier.  Another, better noted in the media, is the appalling number of deaths each year in the desert, as naive crossers from better-watered climes take their coyotes, their smugglers, on faith that the big city lies just across the line.  Women, many dressed in high heels and city clothes, make up a large percentage of the dead.

The Border Patrol’s decision—memorialized as the Southwest Border Strategy in 1994—to restrict illegal immigration to the remotest and most difficult areas had a logic, of course.  Seven years later, the General Accounting Office summarized the strategy as making it “so difficult and costly for aliens to attempt illegal entry that fewer individuals would try.”  As anyone who watched the numbers of crossers climb in the 1990’s in response to sweeping economic changes in Mexico could tell, that goal was never met.  Quite the reverse: Despite expense and hardship, more and more individuals are trying, and more and more are succeeding.  The land bears witness, as remote places such as Organ Pipe Cactus National Monument are overrun by agency decree.

There are remote places, and there are remote places.  Nogales is a supremely busy border port, but it is far enough away from the center of things that it takes a walker ten days to arrive in Phoenix, where a large job market for unskilled workers exists.  That does not stop walkers from making the trek.  Only a few miles west of Nogales, the sheer rock walls of the Atascosa Mountains make a formidable barrier to travel; the lowlands even farther west are laced with river and stream channels that make for far easier passage, even though it is through an unforgiving desert.  Neither geographical fact deters crossers.  Put an overly burdened Border Patrol to watching all that territory, and you are already set for failure—for alambristas, it seems, will take any risk to cross just about anywhere.  All the technology at the agency’s disposal—motion sensors and motion-activated lights, trip wires, closed-circuit televisions, aerial drones—has not lessened their numbers, any more than technology slowed progress on the Ho Chi Minh Trail.  Yet, at least until very recently, the agency seems to have preferred technology to boots on the ground, with the result that, for many years, the border has been essentially open.

The recent crackdown in the countryside has had a few documentable effects.  One is to keep illegal immigrants in, for, when crossing becomes as challenging as it now is, involving not border cities but vast tracts of desert and mountains, then undocumented aliens who successfully enter simply do not leave, as they once did—working for a few months, then returning to their towns and villages to live until the money ran out, then crossing over again.  Having won the alambrista lottery, few take the risk of going home only to find themselves shut out, and so they stay.  The only thing that will keep crossers from the fragile, open desert is to restore border cities as the natural point of passage.  Either that, or build a wall too high to jump, an immodest proposal that has been making the rounds.

Those who come here are, for the most part, unskilled laborers.  Most are poor.  And after their arrival here, most of the unskilled immigrant poor remain poor, working the low-end service jobs that homegrown workers are said not to want.  They drive the substandard vehicles available to them through the secondhand market, vehicles that are minimally maintained and, in the immediate future, are likely to be gas-guzzling behemoths ditched by panicked suburbanites in favor of hybrids and motorbikes.  (Watch, soon, for an appalling increase in the number of traffic fatalities nationwide as these vehicles swell the roads.)  They live in substandard housing whose owners have not found it convenient to install energy-efficient appliances.  That housing lies far from city centers, which are now being reclaimed by two groups of middle-class people: young professionals who find city life congenial and older “empty nesters” who find that being able to walk to grocery stores, pharmacies, and cultural events outweighs home ownership in the distant suburbs.  Many cities are already experiencing white flight in reverse: In Atlanta, in New York, in Washington, in San Francisco, downtown is increasingly a place of wealth, safe and clean, while the distant suburbs are filling with the poor.  One look at the sprawl and ruin that is greater Los Angeles, from the San Gorgonio Pass to the Pacific Ocean 150 miles west, suggests the future of an urbanized nation, a Blade Runner state in which the Darwinian struggle holds center stage.

These are just some of the environmental effects, broadly speaking, of these millions of new arrivals.  But another set of environmental effects caused by illegal immigration along the border, just as troubling, is the product of the government’s response to it.  Faced with the problem of a very long border and too few people to guard it, government agencies of various sorts are busily building a high-tech, multiply cordoned Great Wall along the border—at least at points where access is convenient.  Plans are being made to extend it from ocean to ocean.  One scenario for western Arizona envisions a concertina-wire-topped fence at the international line, followed by a moat, followed by a steel wall, followed by a no man’s land of loose gravel and low-strung wires, followed by another fence, followed by a control road patrolled by robotic vehicles.  (If that sounds to you like the creepiest science fiction, then you are not alone.)  Farther to the west, near San Diego, the Department of Homeland Security has proposed terraforming large tracts of land along the California-Mexico border, filling in canyons, shaving off ridge lines, and raising a triple fence of concrete and barbwire.  The California Coastal Commission rejected that plan, but it is being revised, even as ostensibly conservative U.S. representatives from Southern California have taken the side of DHS against state authorities on the matter of sovereignty.

Certain state agencies in Arizona and New Mexico have called for a less fearsome fence, low but electrified, that would surely be less expensive to build if no less difficult to maintain.  Their sense of fiscal responsibility is laudable, but, as would any barrier, this fence would disrupt another kind of migration: that of wildlife, which knows no international boundaries.  Pronghorn, jaguars, wolves, coatimundi, bighorns—these and many other species require large areas in which to move freely.  Before border policy pushed aspirant migrants into the deserts and mountains, the animals had just that room to roam.  Now their habitat is vanishing.  Any sensible discussion of what to do about the immigration problem will include recognition of their plight, for a homeland of empty ranges and silent trees hardly seems worth defending.

Is there any way to keep that flood of newcomers from crashing on these shores?  Short of a militarized, environmentally neutral border, perhaps not.  Not, that is, unless one of the environmental causes of immigration over the southern border is remedied: the collapse of private agriculture in Mexico as a result of the spectacularly ill-advised North American Free Trade Agreement.  That treaty, signed under Clinton but endorsed by both Bushes, had the effect of making official Mexico’s newfound role as America’s southerly breadbasket, as the provider of fresh produce to New England in the dead of winter, of cheap foods to the rest of the continent at all times of the year.  (The long distance from Mexican fields to Alaskan supermarkets is one reason, of course, that it takes 65 calories of fossil fuel to produce one calorie of food energy for the American market.)  The treaty also helped complete the thoroughgoing industrialization of Mexican agriculture, introducing labor-saving, profit-maximizing machinery while removing people from the land—a sure cause of economic and social catastrophe in any farming community, as development economists will tell you.

In the last decade, those displaced farmers have tended to move in two directions.  Some have gone to the conurbation of Mexico City, the population of which is now unofficially estimated to be 30 million, a great part of which lives in oceanic wildcat slums that stretch from the capital for scores of miles.  Others have gone to El Norte, where, if nothing else, the slums tend to be a touch more livable, the prospects a little better, and the cities less vast, at least for the time being.  I was reminded of this recently when I complained about the size of Phoenix, which has a population of more than three million.  A friend who lives in Los Angeles retorted, “Three million?  That’s fewer than the number of illegals here alone.”

Faced with such conundrums, many environmentalists have chosen to remain altogether silent on the question of immigration, perhaps hoping that it will go away on its own.  Immigration is, after all, a cyclical issue in American politics, one that rises and falls every 10 or 20 years.  I have the feeling, though, that this time it will not disappear: There are too many ruined canyons, tracts of desert land, and streambeds for all that.  So environmentalists must raise it.  With 60 million poor and unemployed and a national debt in the trillions, the American lifeboat may well have sprung one too many leaks to allow more passengers, regardless of where they board.

If it is to take on more, determining just how will require informed, dispassionate debate on many fronts: social, economic, cultural, and environmental.  If it is not, then the question remains how America is to become a fortress without walling herself off in the manner of a Chinese dynasty or Stalinist state, and without our becoming prisoners within.  There are no easy answers.

In the meanwhile, what remains is to protect our garden.