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.

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.

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?”

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.”

---