Donald Trump’s “big, beautiful wall” has become the trademark of his presidency. It is the promise that more than any other has energized his base, and riled his opponents, and his dogged attachment to it has now brought a large part of the US government to a historic 25 days of partial shutdown.

The potency of Trump’s wall – for his supporters and his detractors – stems from its simplicity. Build it tall, build it wide – he has pledged 1,000 miles of it – and America will be safe again.

But how does that uncomplicated notion compare to the complexity of the border itself? Taken as a whole, the 1,954 miles of US-Mexican border is a place of astounding diversity – of terrain, of land use, of city and countryside, of ethnicity. It traverses desert, river, mountain and sea.

There is diversity, too, of political view among the 7.5 million people who live in US border counties. Some are ardent backers of Trump’s wall. Others see their future, and the future of America, as being inextricably linked to that of their neighbor to the south.

Facebook Twitter Pinterest Central American migrant families at a protest at Friendship Park in April last year. Photograph: Carolyn Van Houten/Washington Post/Getty Images

Friendship through the fence

Hike 45 minutes past salt marshes and sand dunes, down a lonely beach empty but for occasional tourists on horseback, and you arrive at a steel fence that juts out into the Pacific Ocean.

This is where Trump would like to start building his wall should he find the billions of dollars necessary. With the impasse over funding, which prompted the shutdown, administration officials have started to describe what’s already here, as well as repairs to a two-mile stretch of fence in Calexico 100 miles to the east, as Trump’s wall. They are not.

The length of wall that has been built by Trump since he entered the White House in January 2017 is zero.

This is the westernmost point of the US-Mexico border, on the outskirts of San Ysidro, California, a suburb of San Diego that is home to one of the busiest border crossings in the world. Here, the hopes of thousands of migrants who try to make it to the US every year are often dashed.

The fence stretches out just to where the waves break, and reaches 15 or 20ft, not the looming 30ft the president has demanded.

Adjacent to this stretch of fence is Friendship Park, a patch of bi-national ground where loved ones from both sides of the border are allowed to meet. The name is paradoxical given the hostility Trump has engendered since he began his wall obsession.

Outside Friendship Park – which only takes 10 visitors at a time – separated families and friends must make do with waving at each other from a distance. Today the US side is unpopulated save for a lone American, and on the Mexican side a father and young boy are looking northwards. “USA!” the man says, pointing through a slot for his son’s benefit.

From San Ysidro, the fence extends for 46 of the next 60 miles of border before finally giving way way to the unforgiving desert. That’s 46 out of a total of 654 miles of fencing that already exists, much of it in various stages of disrepair.

Those hundreds of miles of double reinforced fencing and wire meshing were the product of a different era in politics where some degree of bipartisan consensus was possible. They were largely funded by the Secure Fence Act, an immigration compromise reached in 2006 under former president George W Bush.

Compromise seems unthinkable these days. Trump has laid out a vision of the border that is harshly binary: on his side of the territorial line there is the rule of law, hard work and freedom; on the other side there is criminality, gangs and drug smuggling.

The purpose of the wall, in Trump’s dystopia, is to prevent America being overrun by the dark forces billowing out of its neighbor. In his Oval Office address to the nation last week he said: “Over the years, thousands of Americans have been brutally killed by those who illegally entered our country and thousands more lives will be lost if we don’t act right now.”

Facebook Twitter Pinterest Trump has laid out a binary vision: the rule of law versus criminality, gangs and drug smuggling. Photograph: Carolyn Van Houten/The Washington Post/Getty Images

But talk to people in San Ysidro on the American side of the border, and they will tell you about fear and intimidation inflicted on them by the US government.

In this town, where 90% of residents speak Spanish at home, the land south of the border is not equated with lawlessness and evil, but with family, friends and affordable healthcare. If there’s a dystopia, its not the Mexican one of Trump’s imagining but the hardened militarism that is fast emerging on the US side replete with helicopters, barricades and armed border patrol.

“The feeling like you’re in a war zone is so dramatic the last couple months,” said Lisa Cuestas, head of Casa Familiar, a non-profit that provides social services to San Ysidro.

Militarization sped up after the arrival in Tijuana, on the Mexican side, of the caravan of Central American migrants which Trump made so much of during the November midterm elections, calling it an “immigration invasion”. Now members of the caravan are stuck in Mexico and barbed wire has proliferated everywhere like a mutant weed.

Estrella Flores has family and a job in San Ysidro, working with youth at Casa Familiar, but she lives in Tijuana with her husband and 18-month-old child. Her commute has become hellish since Trump’s border crackdown.

“The first time I went across the border where they had the barricades and the barbed wire and the helicopters I was like: ‘Am I in a war zone? What’s going on here? I’m just trying to get to work.’ This isn’t just a friendly crossing, it could turn very bad, very quickly.”

Such views are commonplace across California. A poll conducted by the San Diego Union-Tribune after Trump’s Oval Office speech found that 56% of Californians opposed the idea of the wall, compared with 34% in favor.

That’s not surprising for a state that is a leading force of progressive politics in the US. But California is also significant for having more undocumented migrants than any other state – 2.4 million to Texas’s 1.7 million.

Close by is the site of Trump’s eight wall prototypes. He came here last March to pose for photos in front of the giant slabs of concrete and steel. Now they languish and rust.

According to a later report by the Government Accountability Office, the eight model sections were riddled with design and construction flaws. US Customs and Border Protection (CBP) tested the slabs and found they can be breached.

Facebook Twitter Pinterest The Cabeza Prieta wilderness near Ajo, Arizona. Photograph: Caitlin O’Hara/Getty Images

Death in the desert

The first light glistens off the frosted spines of the cholla cacti as 30 volunteers in neon yellow shirts fan out to comb the desert under a pale pink sky. Ely Ortiz, the leader of the Águilas del desierto (Eagles of the desert) rallies his team who have driven through the night from San Diego to Ajo, Arizona. He tells them the last time they searched this area they found 11 sets of remains.

The volunteers are joined by a team of cadaver dogs to help in the grim search. A dog called Zabra, whose last job was looking for victims of the California wildfires, is unaccustomed to the desert terrain and has to stop every couple of hundred yards to have barbed spines yanked from her paws.

Unexploded ordnance dropped by the US military in training is one element of danger for migrants.

Trump can already count on a metaphorical wall here. This is the most frequently travelled, but also most deadly, migrant corridor across the Sonoran desert.

Water is scarce and temperatures rise above 120F (49C) in the summer months. The official CBP count records 7,209 deaths along the south-western border in the past 20 years, but that is almost certainly a gross underestimate.

Facebook Twitter Pinterest Indigenous Tohono O’odham people in the Altar desert protest against Trump’s proposed wall. Photograph: Pedro Pardo/AFP/Getty Images

Despite the human tragedy, federal prosecutors have seen fit to prosecute nine volunteers with the humanitarian group No More Deaths. Their offence: “littering” and driving on restricted roads in the Cabeza Prieta reserve when responding to search and rescue calls. The nine will face trial on Tuesday in a federal district court in Tucson.

“The irony is that they tell us we can’t drive here or leave water because it’s protected wilderness, but meanwhile border patrol drives their trucks and ATVs off road, and fly helicopters and drones wherever they want,” one of the nine, Parker Deighan, told the Guardian.

Trump’s policy of “prevention through deterrence” is forcing migrants to take greater risks. As legal admission to the US through official ports of entry becomes ever more restricted, migrants are being “funneled” away from fenced sections of the border towards the desert.

One of the only towns in the area is Ajo. Today it’s a ghost town, as its copper mine closed in the 1980s. Since then most residents have switched to the main local job-provider: border security.

Ajo Samaritans recently gathered in the plaza for a vigil to honor the lives of the people who succumbed in the surrounding wilderness. They laid out 118 white crosses, one for each of those lost in 2018. Those whose bones had yet to be identified were called desconocido – unknown.

Facebook Twitter Pinterest An agent in Nogales, Arizona, watches over the border at dusk. Photograph: John Moore/Getty Images

Making an entry

Before Trump decided to throw a bone to his voter base during the midterm election campaign last year by sending more than 5,000 active-duty military troops to the border, the gate at Lukeville port of entry in Arizona was almost always open. American tourists, seeking to flee the winter, would blithely pour through heading for the beach at Rocky Point, an hour’s drive south on the Mexican coast.

Trump’s border clampdown hasn’t only cramped the style of beach-lovers as they pass through the now half-closed gate. Over the past two months, the US military have brought with them concertina wire and a double stack of shipping containers, ready to be used to block the entry as an impenetrable barricade should another caravan – or “immigrant invasion” – arise.

Not that there’s any sign of that. Most of the traffic through Lukeville is commercial and passenger, and the main concern of federal agents is not migrants but drugs.

It is one of the myths propagated by the Trump administration that America is awash with drugs that have flooded into the country through lack of a wall. In his Oval Office address last week, Trump said that “the border wall would very quickly pay for itself” by halting the flow of illicit drugs – implying that narcotics came into the US through sections of open border.

Not true. In fact, most illegal drugs are hidden away in cars and tractor-trailers as they pass over international bridges and through small ports of entry like Lukeville. The 2018 annual drug threat assessment of the Drug Enforcement Administration points out that for drugs like heroin, up to 90% enters the country through ports of entry.

Facebook Twitter Pinterest The border fence. Trump has pledged to build 1,000 miles of wall. Photograph: John Moore/Getty Images

Another contradiction is that Trump insists on immigrants showing up legally at border crossings, yet for those who do so he has made it increasingly difficult for them to claim asylum. In November the Trump administration announced it would deny asylum to anyone who tries to cross the border illegally, though a federal court has since temporarily blocked the new regulation.

Meanwhile, a new system of “metering” has been introduced that amounts to a federal slowdown at legal entry points. As a result, growing numbers of increasingly desperate families, most coming from the trio of violence-ridden Central American countries – Honduras, El Salvador and Guatemala – have been left stranded in Mexico.

At Lukeville, asylum seekers following Trump’s orders and showing up legally at the port of entry have simply been turned away. The Guardian spoke to Alberto (not his real name) in a shelter on the Mexican side. In November, he presented himself at Lukeville and asked for asylum.

The supervising agent there told him to leave, claiming it was out of hours. Alberto was left on the streets where, days before, mafia members had told him they would kill him if they saw him again.

Volunteer groups working with immigrants protested, and CBP apologized. Now when asked if they accept claims, Lukeville agents repeat the official mantra: “Asylum seekers are being accepted at all ports of entry.”

That doesn’t mean claims will be successful. Alberto did try a second time to have his claim heard. He will testify before an immigration judge in two weeks’ time.

Facebook Twitter Pinterest The skyline of El Paso and Ciudad Juárez, where nearly 3 million people live. Photograph: Joe Raedle/Getty Images

The twin city

There is no place along the border that more strongly rebuts Trump’s dystopian vision than El Paso. The Texas city is so intertwined with Ciudad Juárez across the Rio Grande to the south that they are virtually inseparable.

Between them they are home to almost 3 million people – roughly the size of Chicago. About 20,000 pedestrians and more than 35,000 vehicles cross into El Paso from Mexico every day, many to work, others to go to school or shop.

It’s two-way traffic: Americans in El Paso also regularly cross into Juárez to visit family or experience the nightlife.

Eighty per cent of El Paso’s residents are of Hispanic origin, and a quarter of the city’s population was born outside the US.

Mary Gonzalez, the Democratic representative for El Paso in the Texas house, said: “It’s a very generous, diverse, multinational, welcoming and loving community. That human component is left out when the border is discussed.”

Retail sales in the US side of the twin cities are estimated to generate $10bn a year, with a fifth attributed to Mexican shoppers.

Against that reality, Trump has painted a picture of rampant crime and the threat of violence being imported into the US. That’s a particularly loaded argument for El Paso, given the historically high murder rate in neighboring Juárez.

Facebook Twitter Pinterest Sister Cities, a mural painted by Los Dos, symbolises the bond between El Paso and Ciudad Juárez. Photograph: Paul Ratje/AFP/Getty Images

Last year saw it rise again to almost 200 people killed each month. Yet crime in El Paso on the US side of the border remains relatively low. Violent crimes have fallen sharply from about 6,500 in 1993 to around 3,000 today.

That’s why Beto O’Rourke, the rising star of the Democratic party and former Congress representative for this strongly left-leaning city, lauded El Paso as the “safest city in America” as part of his senatorial election campaign in November. (His claim wasn’t entirely accurate – factcheckers found it was only “half true”.)

O’Rourke failed in his bid to unseat Ted Cruz in the midterm elections, but the fact that he came within three points of doing so suggests that Texans might be more open to his liberal stance on immigration and less in step with Trump than is often assumed.

Opinion polls show that while most Texans are concerned about immigrants entering the country illegally, most are also opposed to Trump’s wall.

Some people go as far as to suggest that El Paso could be a harbinger of things to come for the whole of America. As Josiah Heyman, director of the Center for Inter-American and Border Studies, put it: “El Paso is a place where there is a vision of the future, where people, instead of being part of a closed defensive community, are able to find the joy of relating to others.”

Facebook Twitter Pinterest People cool off in the Rio Grande, just over the border from Laredo, Texas. Photograph: Rodrigo Abd/AP

On the edge – the Rio Grande

Travel 40 miles east from El Paso along the Rio Grande to Tornillo, and it feels a world away from the big city. Vast open areas are filled with orchards, and pecan nut and dairy farms.

US border patrol vehicles keep a watchful eye.

Of the 1,317 miles of border with no fence or human barrier, much of it lies along the 1,248 miles of Rio Grande. The river serves as the demarcation line between the two countries all the way from El Paso to the Gulf of Mexico.

Powerful currents, towering canyons and cliffs that in the Big Bend section rise to 50ft reduce the need for a solid barrier through much of the river’s trajectory.

Still, Trump has his eye on the Rio Grande for his wall. Of the $5.7bn he is demanding from Congress, a sizable chunk would go towards building more than 100 miles of wall along the river.

Here just outside Tornillo, the existing fence that runs west to Sunland Park in New Mexico has a 10-mile gap in it. Despite the lack of an artificial barrier, most locals go about their daily routine without fuss.

Miguel Alvarez, who has lived close to the border for 25 years, said the arrival of the fence has made precious little difference. “You still see small groups of people passing through just like they did before any fencing,” he said.

A farmer who didn’t want to reveal his name said he would feel more comfortable if the gap in the fencing were closed. “I haven’t had any issues with people coming across, but you never know,” he said.

Tornillo itself fell under the national spotlight after the Trump administration chose the Marcelino Serna port of entry south of the town to house thousands of unaccompanied minors in a tent-like facility. It was meant to be temporary, designed to help deal with hundreds of children who had been separated from their families as a result of Trump’s crackdown on border crossers.

But it rapidly grew, to up to 2,400 beds, becoming the face of the brutality of Trump’s policy of tearing families apart as a form of deterrent.

In November, a government watchdog warned that conditions in the tent city were putting children at risk, and since then the numbers have been reduced until the last teenager was transferred out of the facility last week. As far as locals are concerned, its closure couldn’t happen fast enough.

They are eager for things to get back to normal. Or at least as close to what passes as normal these days.

• This article was amended on 15 January 2019 because the fence is not unbroken for the 46 miles from San Ysidro to the desert, as an earlier version said.