“It’s scary, to be honest,” said Elaine Doherty, 50, a psychologist and fellow activist in the campaign, which formally calls itself Derry Girls Against Borders. “Brexit is just months away — and there’s not a single person who can tell you what will happen to us.”

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The 310-mile border that cuts across the island of Ireland has become perhaps the single greatest impediment in the divorce negotiations between Britain and the European Union.

“A real sticking point,” as British Prime Minister Theresa May put it.

The challenges loom over how to continue to allow for the free movement of people and trade between the Republic of Ireland, which will remain in the European Union, and Northern Ireland, which will leave along with the rest of the United Kingdom.

And how to keep the border just as invisible, even as the U.K. and the E.U. inexorably diverge — each free to establish their own immigration controls, customs tariffs and food safety rules.

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And finally, how to do all this without upsetting the delicate peace in Northern Ireland that has relied on an open border.

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People north and south are quick to say there will be no returning to the “Troubles” — the vicious, intimate guerrilla war between pro-British Protestant unionists and Irish Catholic republicans that left more than 3,500 people dead.

Yet, sectarian lines remain deeply drawn in Northern Ireland. Many people in this border city — still known as Londonderry by Protestant residents and Derry by the 75 percent with Irish Catholic heritage — worry that a bungled Brexit could rekindle tensions and possibly lead to violence.

Today, driving along the Irish border, you might pass a farmer who has a barn in one country but grazes his sheep in the other. Almost 1 million people freely cross the squiggly line on the map each month. There are 200 official crossing points, and nobody knows how many dirt roads, foot trails and cow paths. The economies are tightly intertwined.

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Border checkpoints, and all the militarized infrastructure of barracks, watchtowers, bunkers and blast walls, were removed from the island of Ireland in the aftermath of the 1998 Good Friday Agreement, a hard-won pact that ended 30 years of violence. The deal was in many ways a masterpiece of diplomacy — it didn’t seek to resolve all political differences in Northern Ireland but instead acknowledged the “continuing, and equally legitimate, political aspirations” of republicans and unionists.

E.U. membership made such evasion possible. E.U. policies of free movement and free trade allowed Northern Irish republicans to feel more connected to the Republic of Ireland, while unionists could continue to be an integral part of the United Kingdom. No one had to choose. Lines, grievances, identities could begin to soften.

But after Brexit?

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Republicans worry that a defined border on the island would undercut their relationship with the rest of Ireland. Leaders of Sinn Fein, the republican political party, have warned that any Brexit border would hasten the day they seek an island-wide vote to unify.

British loyalists are livid about the E.U. proposal to place a customs border in the Irish Sea between Northern Ireland and Britain. May’s governing partners, Northern Ireland’s hard-line Democratic Unionist Party, oppose any kind of “special status” that would make them separate from the United Kingdom.

European Council President Donald Tusk has blamed the Brexit campaigners, “who are 100% responsible for bringing back the problem of the Irish border,” he said in a tweet.

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In the 2016 referendum, 56 percent of Northern Ireland voters cast their ballots to remain in the European Union. In Derry, it was 78 percent.

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“Brexit has re-politicized everything,” said Jennifer McKeever, president of the Londonderry Chamber of Commerce and owner of a shuttle bus service with a third of its staff and customers living across the border.

May and her European counterparts have promised there will never again be a hard border on the island of Ireland. But what’s a real-world and politically feasible alternative? They haven’t said, because they don’t know.

May vows that her negotiators in the transition period after Brexit begins in March will craft an unprecedented free-trade accord with Europe that makes an Irish border unnecessary.

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Failing that, the prime minister says, Britain will deploy a not-yet-invented “technological fix” — perhaps a system that employs cameras with facial recognition software, plus mobile tracking apps and customs checks in warehouses far from the border.

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Northern Ireland’s top police officer has warned that any customs posts or security installations would be viewed as “fair game” for attack.

“The last thing we would want is any infrastructure around the border, because there is something symbolic about it and it becomes a target for violent dissident republicans,” Chief Constable George Hamilton told the Guardian newspaper.

Paddy Gallagher, 26, is a spokesman for a new fringe political party called Saoradh, which means “liberation” in Irish. The group is home to hard-line republicans who reject the Good Friday Agreement.

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Gallagher concurred that “any sign of a fixed border” would quickly become a target. A remote camera recording license plates? A customs collector with bar-code scanner? “Capable groups would be willing to attack them,” said Gallagher, careful not to endorse violence himself.

Saoradh’s headquarters in Derry were raided in October by police anti-terrorism units, which confiscated 330 fireworks.

People here point to a disturbing week of violence this summer, sparked by unionist parades celebrating the “Twelfth of July,” the victory of Protestant Prince William of Orange over the deposed Catholic king James II in the Battle of the Boyne in 1690.

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Angry crowds in the Catholic Bogside neighborhood erected barricades to shut down streets. Although the protests were dismissed by many as “recreational rioting” by drunken mobs, more than 70 petrol bombs were hurled, alongside two pipe bombs thrown at police officers.

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This civil unrest occurred on the same streets that were the backdrop of “Bloody Sunday,” when British soldiers shot and killed 14 unarmed protesters at a civil rights march in 1972.

“It was in Derry where the Troubles started, and it was in Derry where they ended, too,” said Brenda Stevenson, 51, a former mayor here and the niece of the Irish leader John Hume, who shared a Nobel Peace Prize in 1998 for his role in ending the conflict.

Today, there’s fresh graffiti in the Bogside urging the young to “Join the IRA” — even though the Irish Republican Army retired its armed campaign in 2005.

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A short walk away, in the Waterside, the shrinking Protestant enclave, a mural proclaims that “the loyalists are still under siege.”

Time is running out on the Brexit negotiations. Frustrated by British delays, the Europeans have insisted on a “backstop,” a legally binding insurance policy, to preserve an Ireland without borders in the event that a free-trade deal eludes them. In that case, Northern Ireland would remain a member of the E.U. customs union until the issue is resolved — a proposal that May has, until now, rejected.

Along the Irish border, people worry that the British prime minister might abandon her commitment to open borders. They have good reason. Polling commissioned by the University of Edinburgh and Cardiff University found that 75 percent of English Tory voters would accept the collapse of the Northern Ireland peace process “as the price of Brexit.”

Protestants and Catholics in Northern Ireland think they are not taken seriously. Both sides were flummoxed to learn that Karen Bradley, May’s new secretary for Northern Ireland, was so ignorant about Irish politics that she confessed surprise in an interview that “people who are nationalists don’t vote for unionist parties and vice versa.”

Nor were they happy to hear former foreign secretary Boris Johnson say that a soft border in Ireland would be no different from the discreet cameras that record and later charge drivers congestion fees for vehicles entering central London.

Jeanette Warke, 74, founded the Cathedral Youth Club for children in Protestant Waterside in 1972 with her husband, David, now deceased. They were worried about youths joining paramilitary groups.

Warke said she voted to leave Europe, although “we were not clear what Brexit was about.”

She said, “You voted to ‘leave’ if you were Prod [Protestant] and ‘remain’ if you were Catholic, was the way it seemed to me.”