The emotional core of the policy known as Brexit — shorthand for a British exit from the European Union — is not a national obsession with the details of trade policy. There are no passionate debates in pubs up and down the land over customs tariffs.

For most people, Brexit is about something more visceral: national identity. The perceived need to “take back control” over Britain’s borders, and sharply limit the number of foreigners permitted to live and work in the country, was endorsed by a narrow majority of voters in the 2016 referendum. For nationalists, Brexit is a simple choice to withdraw from an economic bloc that effectively erases the borders between member states by requiring the free movement of people, as well as goods and services.

What few Brexit supporters on the British mainland seemed to realize at the time, though, is that their country, the United Kingdom of Great Britain and Northern Ireland, has just one land border with the EU, and it is a highly contested one — the line of partition that the British Empire imposed on Ireland a century ago. For the past 20 years, it has been easy to forget about the mayhem and death caused by the imposition of that border, since joint membership in the EU facilitated a peace agreement that stopped the bloodshed in Northern Ireland and removed the need for security and customs checks along what one Irish writer has called the “line of malice” dividing Ireland.

NEWRY, NORTHERN IRELAND - MARCH 29: A former customs guard hut directly situated on the north south Irish border stands disused as Brexit is triggered on March 29, 2017 in Newry, Northern Ireland. The northern Irish border is the United Kingdom's only land border with the rest of Europe. British Prime Minister Theresa May will address the Houses of Parliament later today as Article 50 is triggered and the process that will take Britain out of the European Union will begin. (Photo by Charles McQuillan/Getty Images)

A disused former customs post situated directly on the open border outside Newry, Northern Ireland.

Photo: Charles McQuillan/Getty Images

For all the talk of a British exit, however, it is important to understand that Brexit was primarily an English decision: Eighty-seven percent of the votes in favor of leaving were cast in England, and two-thirds of those who consider themselves more English than British voted to leave.

While English nationalists rallied around nostalgia for a British Empire they once dominated, majorities in two other constituent parts of the U.K. — Scotland and Northern Ireland — voted against Brexit, setting the stage for one or both to eventually leave the union.

That support for Brexit — like support for the British Conservative party — comes mainly from England helps to explain why talks with the EU over the country’s withdrawal are currently deadlocked over an issue that English nationalists seemed all but unaware of until recently: the frozen but still unresolved conflict in England’s first colony, Ireland.

Had English rulers never embarked on the centuries-long process of colonizing Ireland, or never imposed a partition on the island in 1921, to create a loyalist enclave where British Protestant settlers outnumbered the native Irish Catholics, it would today be a relatively simple matter for Britain, encompassing just England, Scotland, and Wales, to exit the EU.

Instead, Prime Minister Theresa May now finds herself locked in complex negotiations over how to extract the whole of the U.K. from Europe’s single market and customs union without undermining the fragile peace in Ireland. That’s because the customs and immigration checks that English nationalists see as a minor inconvenience, worth tolerating to live in a less multicultural society, would require border infrastructure along the old partition line in Ireland, causing a range of headaches for Irish businesses and commuters, and making a return to violence all but guaranteed.

At the annual Cross Border Organized Crime Seminar in Ireland last year, police chiefs from both parts of Ireland warned that “the more infrastructure there is at the border, the greater opportunities that creates” for violence by political dissidents and former paramilitaries already involved in smuggling.

Unlike many of the Brexit hard-liners in her party, the British prime minister does at least seem aware of the fact that Britain has a moral and legal responsibility, under the terms of the 1998 peace agreement, to prevent the bloodshed that would likely follow any move to repartition Ireland by re-establishing the border.

May also finds herself in a bind because the European Union, which takes its role in preventing conflict on the continent seriously, has insisted that no talks on a future trade relationship with the U.K. can even begin until she first signs a withdrawal agreement guaranteeing that the border with Ireland will remain open.

That’s easier said than done, however, because the logic of Brexit, and of the European Union, demands a closed external frontier.

Then there’s the problem that the winding, 300-mile-long frontier defined as the border of the new province of Northern Ireland by the British government in 1921 was in no sense a natural border. There is no mountain range or body of water dividing Ireland in two parts, there is just a line on the map, hastily drawn by retreating colonial civil servants — a strategy later termed “divide and quit” by Penderel Moon, a British colonial officer involved in the empire’s equally ill-conceived partition of India in 1947. (Four decades later, the British viceroy who oversaw the bloody partition of India, Louis Mountbatten, was killed by the I.R.A. while vacationing in Ireland.)

The 1921 partition line that created the new, British-controlled province of Northern Ireland, along ancient county borders that these days run through fields, towns and houses, was also drawn in the service of a blatant act of large-scale gerrymandering. It was a new line drawn on the world map to divide one island nation into two parts so that a retreating colonial power could ensure that the descendants of its settlers would control an enclave in which they would be in the majority.

That division gave rise to decades of political violence and terrorism in Northern Ireland and Britain, known with Ulster understatement as “the Troubles,” during which more than 3,500 people were killed, basic civil rights and due process were suspended, and the British Army erected watchtowers and checkpoints and destroyed hundreds of roads, bridges, and country lanes to control the flow of people and goods between the two parts of Ireland.

For anyone who grew up in the shadow of that border in Ireland — I spent childhood summers visiting my mother’s family on either side of it — it is impossible to forget the emotional impact of the hours spent waiting at one of the few authorized crossing points, the tense encounters with armed soldiers at the heavily fortified British Army checkpoints. And yet, for the past two decades, under the auspices of the EU, security and customs checkpoints have been removed, and the border between the two jurisdictions on the island is now as scarcely perceptible as the line between any two American states. It is like the disappearance of a scar.

NEWRY, NORTHERN IRELAND - MARCH 29: The point where north becomes south along the Irish border is marked by the road markings turning from yellow (south) to white (north) are seen as Brexit is triggered on March 29, 2017 in Newry, Northern Ireland. The northern Irish border is the United Kingdom's only land border with the rest of Europe. British Prime Minister Theresa May will address the Houses of Parliament later today as Article 50 is triggered and the process that will take Britain out of the European Union will begin.

Thanks to a peace deal facilitated by the European Union, the once fortified border between Ireland and Northern Ireland has all but disappeared, replaced by subtle markers, like a different color of paint used to mark the edge of a road.

Photo: Charles McQuillan/Getty Images

Watching the Brexit talks from Ireland, Denis Bradley, a journalist and former vice chair of the police board for the Police Service of Northern Ireland, observed that both the U.K. and the EU have underestimated the determination of the Irish people who suffered through the years of violence to tolerate a border. “The Border doesn’t need a solution because it is already solved,” Bradley wrote in the Irish Times.

“Twenty or so years ago the Irish Border disappeared,” Bradley explained. “The old customs posts had long disappeared and then, one day, the British army lifted its gear and went home. Most of the people felt a burden lift off their shoulders — a people who had lived in the shadow of its presence were, for the first time, free of the inconvenience and the scar on the landscape. Since then they have lived with that freedom, and they have judged it to be right and good, and they have no intention of giving it up.”

“The question of the Irish Border needs to be isolated from the sphere of economics and considered in the context of fundamental rights,” Eoin McNamee, a novelist and screenwriter who grew up crossing the border on his way to school, wrote last year. “Theresa May can insist that her country leaves the European Union. The EU can dictate what conditions it wishes. But neither of them can dictate or insist that a line of malice be redrawn across this island.”

The reality of that time, McNamee reminded readers, was “cratered roads, corpses dumped in black plastic bags, surveillance towers, bullet-riddled gospel halls, bullet-riddled homes. You got the full force of it at night. Driving on the empty roads through eerie zones emptied of all except watchfulness and ill-intent.”

I spoke with McNamee in London last week. “The economics are not the key matter,” he told me. Partition, he said, was “a moral wrong, in the sense that the Berlin Wall was a moral wrong, an affront to civilization.” A decade after the border infrastructure had been removed in the wake of the peace deal, and the scar tissue had begun to heal, McNamee recalled his brother suddenly turning to him to ask, “Was that all a dream?”

British Army Engineers dismantle the Watch Tower at Newtonhamilton, Northern Ireland after the armoured Sangar was uprooted by crane. The British government has started dismantling military watch towers across Northern Ireland.  *... in response to the announcement by the IRA that they have begun to decommission their weapons.   (Photo by John Giles - PA Images/PA Images via Getty Images)

British Army engineers dismantled a watchtower in County Armagh, Northern Ireland in 2001, following the peace accord.

Photo: John Giles/PA Images via Getty Images

For the U.K. or the EU to insist on the repartition of Ireland, McNamee says now, would be like telling the German people that the Berlin Wall has to be reconstructed. “It’s not just that people don’t want it — they don’t want it — but they can’t go back to it.”

Or, as the Irish comedian Andrew Maxwell put it recently when asked if there was a solution to the Irish border question: “It’s not the Irish border — it’s the British border in Ireland. The Irish border is the beach.”

Unlike pro-Brexit nationalists in her Conservative party, May seems desperate to strike some sort of deal to keep the whole of the U.K. as close to the EU as possible, not just to prevent a return to bloodshed in Northern Ireland, but to protect British industry. For that reason, earlier in the negotiations over the terms of the U.K.’s withdrawal, May seemed willing to accept an offer from the EU to protect the peace in Northern Ireland by granting the region special status after Brexit, allowing it to remain both in the U.K. and in the European customs union and single market, making border checks for goods and people traveling to and from Ireland unnecessary.

Unfortunately, however, while May signed off in principle to this deal late last year — at least as a “backstop” that would come into force only if the U.K. is unable to negotiate a future trading relationship with the EU that is so close as to render border checks unnecessary — she is now too politically weak to convince the rest of her party to go along.

Perhaps even more importantly, with no parliamentary majority after her disastrous decision to call an early general election last year, the prime minister also needs the support of 10 Northern Irish members of Parliament from the Democratic Unionist Party, a group of Christian fundamentalists who campaigned for Brexit, but against the 1998 peace agreement. While the DUP pays lip service to the idea of keeping the border open, they have threatened to bring down May’s government if she makes any move to accept special status for the region, which would involved customs checks on goods moving to and from Britain. The party’s leader, Arlene Foster, recently described her red line against any such concession as “blood red.”

The halting, drawn-out talks over Brexit have created a deep sense of exasperation shared by supporters and opponents of the policy. In late June, when two full years had passed since Britain had voted to leave the European Union, but there was still no sign of what that would mean in practice, one English soap opera star had had enough. “Who knows about Brexit? No one’s got a fucking clue what Brexit is,” actor Danny Dyer told a stunned Piers Morgan during a television talk show.

“No one knows what it is,” Dyer added, as the camera cut to his fellow guests, Pamela Anderson and Jeremy Corbyn. “It’s like this mad riddle that no one knows what it is.”

The viral success of the clip owed much to the fact that Dyer went on to call the former prime minister, David Cameron, a “twat” for having called the referendum and then “scuttled off” to enjoy a nice retirement “in Europe, in Nice, with his trotters up” after his side lost. But the actor also voiced a frustration shared by millions of Britons who voted for the decision to leave the EU, only to discover that their political leaders had no actual plan for how to do it.

The reason for the long delay is that Cameron’s successor, May, has indeed been trying to solve a riddle that just might have no answer. But the fact remains that more than two years after she took power with the vague pledge that “Brexit means Brexit,” the prime minister has still not spelled out exactly what sort of future relationship she wants with the EU or how she plans to both take full control of the country’s borders without threatening the peace in Ireland.

She initially promised to deliver two things. First, there is the clean break with Europe that the anti-immigrant far right of her party demands — leaving both the customs union and European single market. Second, May promised to negotiate a future trade relationship with the remaining EU members that would satisfy Conservative moderates who worry that leaving the customs union and single market could destroy a British economy deeply entwined with the rest of Europe.

But a third requirement is proving itself to be the most problematic: May is constrained by her promise to find some way to pull the entire U.K. out of the EU without jeopardizing the fragile peace along that previously contested and militarized border.

In a recent Twitter address to the leader of the Brexit campaign, Boris Johnson, Patrick Kielty, a comedian from Northern Ireland whose father was murdered by paramilitary gunmen during the Troubles, explained that the EU was a key component in the “devious magic” of a peace deal by which mainly Protestant “Unionists were guaranteed that Northern Ireland would be part of the U.K. until the majority voted otherwise,” and “the border was removed and the island linked” so that mainly Catholic Irish “Nationalists could pretend they were already living in a United Ireland.”

“Some of these Nationalists then accepted being part of the UK as their day to day lives were essentially Irish,” Kielty continued. “This cunning plan was sold to us on the basis that we were all part of the EU therefore fixation on nationality was so last World War.”

By removing the EU from this spell, pro-Brexit politicians “have opened a Pandora’s box for Northern Ireland,” Kielty added. “It’s one reason why the majority of people in [Northern Ireland] voted to remain in the EU”

Experts like R. Daniel Kelemen, a professor of political science and law at Rutgers University, have tried to alert May to the fact that there is only one way to keep all three of her promises: “magical thinking.”

Determined to press on regardless, May has continued to float proposals that satisfy no one and have raised fears that the U.K. could run out of time to strike a deal before the country’s membership in the EU expires at 11 p.m. on March 29, 2019. If that happens, a so-called No Deal Brexit could disrupt everything from the flow of food and medicine into the country to air travel out of it.

That’s led to demands from the EU for May to simply embrace the fallback plan she had agreed to last year, which would grant special status to Northern Ireland after Brexit. Under pressure to satisfy the hard-liners in her own party and the DUP, May angrily rejected the same plan she had agreed to last December as an attempt to divide her country in two, ignoring the fact that Northern Ireland exists because Ireland was divided in two by Britain in 1921.

As May’s former aide Matthew O’Toole pointed out in The Spectator last month, what the prime minister fails to acknowledge is that the peace agreement signed in Belfast on Good Friday in 1998 already gave Northern Irish citizens special rights that will tie them more closely to the EU after Brexit. As a result of the peace agreement, all citizens of Northern Ireland are entitled to hold Irish and U.K. passports, which means that after Brexit, it could be a region of a non-member country entirely populated by EU citizens. The Good Friday Agreement also gave the region permission to secede from the U.K. and join the Republic of Ireland if a majority of voters ever approves that change in a referendum. So, O’Toole notes, “It is the only part of the United Kingdom where citizens have both a permanent legal right to E.U. citizenship, and a formal route back to E.U. membership via a future referendum on Irish unity.”

Whatever the outcome of the talks, many Irish observers have been stunned to see how blithely their concerns about what would amount to the effective repartition of Ireland have been dismissed by May’s government and the hard-line, self-described “Brexiteers” pushing her party to the right.

One of the leaders of the arch-conservative faction, Jacob Rees-Mogg, has consistently downplayed the risk of chaos if Britain crashes out of the EU without a deal. He has attracted particular derision in Northern Ireland — where a majority voted against Brexit — for saying that there was no need for him to visit the 300-mile-long Irish border, which might be sealed after Brexit to prevent British goods from being smuggled into the EU.

Rees-Mogg argued that he gets all the information he needs on the border from members of the DUP, whose aim is to keep Northern Ireland inside the United Kingdom. However, none of those Northern Irish MPs actually represent communities along the border with Ireland, which all voted decisively against Brexit in the 2016 referendum.

“He’s a very good example of the cocoon that ardent Brexiteers are living in — he knows nothing about the Irish border, he has no interest in it,” Deirdre Heenan, a professor of social policy at Ulster University, said recently of Rees-Mogg. “The man is living in denial; living in some sort of deluded dreamland that post-Brexit, we’ll go back to a grand, imperial Britain.”

Over the summer, video circulated of Rees-Mogg admitting in an offhand way that Brexit might require a return to the sort of border checks carried out during the 30-year civil war of the Troubles.

“There would be our ability, as we had during the Troubles, to have people inspected,” Rees-Mogg said at a seminar. “It’s not a border that everyone has to go through every day, but of course for security reasons during the Troubles, we kept a very close eye on the border, to try and stop gun-running and things like that.”

The Conservative member of Parliament — who seemed concerned only with preventing European immigrants from crossing into the U.K. via Northern Ireland — sounded unaware of just how intrusive and provocative those searches were. For the three decades before the 1998 peace agreement in Northern Ireland, which gave all residents the right to both U.K. and Irish passports, most cross-border roads were closed by the British Army, and every car that did cross was inspected at heavily fortified checkpoints, first by armed soldiers and then by customs officers.

Ireland’s foreign minister, Simon Coveney, reacted with horror to the Conservative politician’s ignorance of how traumatic, and deadly, the recent past was in Northern Ireland. “It’s hard to believe that a senior politician is so ill-informed about Ireland and the politics of the Brexit Irish border issue that he could make comments like these,” Coveney observed on Twitter. “We have left ‘the troubles’ behind us, through the sincere efforts of many, and we intend on keeping it that way.”

Last month, there was a more poetic, but equally jagged response to Rees-Mogg and others who have dismissed the hazard of reimposing the border from the Northern Irish playwright Clare Dwyer Hogg and the Belfast-born actor Stephen Rea in a lyrical, Financial Times opinion piece filmed along the border.

“Jacob Rees-Mogg you’re right. You don’t need to visit the North of Ireland to understand the border — you need to have lived here,” Rea says, standing at the border. “We live here, and we’re holding our breath again.”

Correction: Wednesday, Oct. 24, 12:22 p.m. EDT
An earlier version of this article incorrectly identified the Irish comedian Andrew Maxwell as Northern Irish. Although Maxwell has relatives in Northern Ireland, he is from Dublin, which his accent makes abundantly clear.