The wall is an army in brown. It is fabricated in sections ten girders wide, eighteen feet tall and crowned with a metre-high blade. To watch the slatted world on the other side, Mexico, as you walk through the city of Nogales, is to be reminded of a zoetrope’s flickering image; the same sequence played again and again. The steel, untreated, is red-brown with rust, and this rust in turn has leached into the wall’s concrete base and drained down its sides to the ground.

The wall divides the town – Nogales Arizona/ Nogales Sonora – though most of the population lives on the Mexican side. On one of the slopes on the US side is a shrine. Ranged along a reinforcement joist slanting from the wall’s concrete base are some burnt-out tealights in glass jars. Knotted to the vertical palings above are a length of curled yellow ribbon and, tied in place with the same kind of ribbon, a bunch of dirty plastic daisies turned brittle by the sun. Nogales Sonora, on the other side, is twenty feet below, and I realise that the wall stands on its own embankment – steep on the Mexico side, like a castle dyke. In order to climb the wall from Nogales Sonora you first have to climb the slope. About thirty-eight feet, all told. Through the wall, in Mexico, I can make out a white, windowless building and a sign: despacho juridico, legal office. Stencil-sprayed on the adjoining wall, a young man’s face – a boy’s really, in its chubbiness – repeated over and over, like a crude Warhol, like a picture of a martyr.

José Antonio Elena Rodríguez died in Mexico; the bullets that killed him were fired here in the United States. It happened on an October night in 2012. Border Patrol had been called to a report of men climbing the wall. As the agents converged, the men climbed back over to Nogales Sonora. A crowd gathered on the Mexican side and began throwing rocks, over the fence, at the patrolmen. Among the rock-throwers was José Antonio Elena Rodríguez (this is the official version). José Antonio Elena Rodríguez threw no rocks, he was merely walking past the fence on the way home from basketball (the unofficial version, the version told by Rodríguez’s friends and family and other civilian witnesses on the Mexican side). The Department of Homeland Security has declined to release its video footage of the incident. It would compromise national security. Everyone agrees that José Antonio Elena Rodríguez was sixteen.

On the twenty-foot-high bluff, behind the eighteen-foot-high fence, stood the eight Border Patrol agents. Among them was Lonnie Swartz. At the foot of the bluff was José Antonio Elena Rodríguez, walking home from basketball, or throwing rocks – both, perhaps – throwing rocks over the top of the fence, thirty-eight feet above him. Again there is no question that Lonnie Swartz approached the fence, and drew his pistol, and shot down on José Antonio Elena Rodríguez, hitting him ten times from behind, pausing only to reload.

The federal investigation rested not only on whether Swartz’s actions were reasonable – he feared for his life, went the defence, rocks big as pomegranates raining down – but whether the killing could even be described as criminal when the kid was a Mexican in Mexico, and therefore exempt from the protections of the US Constitution, and his killer an American in America.

A man stands on an alluvial fan below Tumacacori Mountain, a few dozen miles north of here. His name is José, too: José Salazar Ylarregui. This is 1851. He is a senior member of the Joint United States and Mexican Boundary Commission, responsible for surveying the newly defined southern border. Until 1851 there was no line, no wall. It was war that created the line, the Mexican–American War of 1846, or rather the Treaty of Guadalupe Hidalgo that was the war’s culmination. Salazar, jointly responsible for some hundred men, in the unspeakable heat of the Sonoran Desert, on constant guard against Apaches, made a note. Maybe he imagined his superiors in their cool Mexico City drawing rooms: ‘On paper one easily draws a line with a ruler and pencil.’

In the east, that line followed the natural barrier of the Rio Grande River; in the west it travelled, originally, from El Paso along the Gila River to its confluence with the Colorado, and thence in a straight line to San Diego Bay, placing much of today’s southern Arizona in Mexico. The Gadsden Purchase of 1853 caused the western portion of the line to be repositioned further north. From then on the border, rather than following the course of the Gila, was a straight line from El Paso to the Colorado. Upon a stretch of desert where there were few natural features, and fewer names on the maps of either nation, significance was conjured.

The nature of the border did not escape those tasked with making it a reality. One member of the 1851 survey, observing the newly designated borderlands, asked simply: ‘Is this the land we have purchased, and are to survey and keep at such cost? As far as the eye can reach stretches one unbroken waste, barren, wild, worthless.’ Another described a ‘sterile waste, utterly worthless for any purpose than to constitute a barrier’. Travelling in the region in 1909, the Norwegian explorer Carl Sofus Lumholtz wrote that ‘the sensation was that of walking between great fires’. Until the twentieth century the border was little more than notional: a line on paper echoed by a string of obelisks on the ground, each one separated from its neighbours by two miles of desert; further where conditions were harshest. The statesman’s desert, from the Chinese Taklamakan to the Libyan Sahara: a bulwark, a buffer. But more than that, a weapon for turning your enemies to dust.

Since the 1990s a doctrine known as ‘prevention through deterrence’ has been practised by Border Patrol. Even today, only 351 miles of the 1,954-mile border are effectively fenced – 18 per cent. Increasingly, Border Patrol is deploying remote electronic surveillance technology, the so-called ‘virtual wall’ – infrared cameras, motion-sensors, radar, drones, blimps – but in many places you can still pass from the southern side to the northern with a single step, even if that step must be flanked by a hundred thousand others. Eight kilometres east of Nogales, where the Santa Cruz River crosses (or is crossed by) the border, the fence simply stops, and all one need do in order to move from one country to another is edge along the river’s bank.

The militarisation of the borderlands was accelerated following the terrorist attacks of 2001 and the creation of the Department of Homeland Security. By obliging those who are determined to cross illegally in the harshest areas – the Organ Pipe Cactus National Monument west of Nogales, say – the strategy became one whose efficiency can be measured not only by the number of would-be migrants discouraged from embarking (which is anybody’s guess), but by the number of human remains recovered from arroyos and the shadows of ironwood trees. Between October 2000 and September 2014, in southern Arizona alone, this number was 2,721. Among these people – who succumbed to heatstroke or dehydration, or fell from cliffs or died of snakebite or heart attacks – some eight hundred are unidentified. To this number may be added those remains that have not been found, either because of their remoteness or, more likely, because they have simply been erased.

Like most deserts, the Sonoran is walled in by mountains: to the north and east are the Colorado Plateau, the Rockies and the Sierra Madre; to the west the Sierra Nevada. It is the Sierra Nevada that is responsible for much of south-west America’s aridity, removing the moisture from the Pacific’s clouds as they are drawn up its western flank. They call it a rain shadow, but the effect is not so much a shadowing as a milking. It is from the low-lying south – the tropical south of central America – that the monsoon comes, and the result is a desert that, while being dry enough to kill dozens of people each year, can feel deceptively abundant.

Sometimes the desert preserves – 2,000-year-old mummies have been found under the sands of the Taklamakan, their tongues still pink – but more often, it obliterates. Those shapes helixing high above, shuddering on their huge wings, are turkey vultures, and with the coyotes and the foxes they will strip a body of meat and disperse its bones over a square mile in the course of a few days. As you wait on the Mexican side of the border before trying to enter the desert, therefore, you do so in the knowledge that it is not just your life that you are staking, but – in the absence of your corpse or, if your corpse is recovered, any way of identifying it – your loved ones’ opportunity to grieve for you.

After 9/11, the Department of Homeland Security evolved a new method of ‘deterrence’. It was called ‘enforcement with consequences’. Until then, those apprehended in the desert were processed and dispatched to the nearest port of entry without prolonged detention. This was called ‘voluntary departure’, because the migrant waived his or her right to a judicial hearing. Naturally, once removed from the country, they attempted to cross again as soon as possible – the next day, why not, since they’d come this far. ‘They know the game,’ said a patrolman interviewed at the time. ‘They’re delayed eight hours and then they do it over again.’

Part of ‘enforcement with consequences’, since 2005, is Operation Streamline. It is here that the element of deterrence comes in. Instead of being allowed to leave the US under the terms of ‘voluntary departure’, apprehended migrants are instead processed through the federal criminal justice system. Under Operation Streamline, first-time offenders can be sentenced to up to six months in jail, repeat offenders up to two years.

The Remains of the Day