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The status of the Irish border, which was a flashpoint during the 30-year conflict known as "The Troubles," threatens to derail Brexit talks.

John Sheridan stood ankle deep in the lush grass of his farm tracing the border between Northern Ireland and the Irish Republic with his finger across the skyline.

"I'm being shoved back into a corner," he said, following its path in a 270-degree arc.

"There's a young lad there," he said, waving in the direction of his 22-year-old son, Chris. "He doesn't remember what it was like when there was a border."

No memories of the soldiers, the searches, the checkpoints to be traversed to enter towns right across the border.

"It was soul-destroying," Sheridan recalled.

Like many living in communities along the border, Sheridan is adamant that Britain's looming exit from the European Union must not bring the reinstatement of a physical boundary, including the return of manned customs posts or security cameras. Some in both Northern Ireland and the Irish Republic fear Brexit would rekindle tensions and even spill over into violence.

As President Donald Trump pursues his pledge to build a wall along the U.S. border with Mexico, and several Eastern European nations have constructed border fences to keep out migrants, the West appears to be re-embracing the idea of establishing boundaries.

But it isn't quite so straightforward here.

For much of the late 20th century, the border was a front line in the bloody 30-year conflict known as "the Troubles." The dispute pitted republicans, primarily Catholics seeking a united Ireland, against security forces tasked with maintaining British rule over Northern Ireland. They also fought against local loyalists, primarily Protestants wanting Northern Ireland to remain in the United Kingdom. Some 3,600 were killed - with groups on all sides among the perpetrators and victims.

A delicate peace was negotiated in 1998, and the military checkpoints and watchtowers that had been regularly targeted by republicans as symbols of British dominance gradually disappeared.

Today the border is more or less invisible, with little more than a change in the asphalt and different road signs indicating that you have left one country and entered another. There are more than 200 official - and innumerable unofficial - crossing points.

There is a saying in Northern Ireland that whoever drew the partition line must have been on the whiskey, as it twists-and-turns seamlessly through farmland, schoolyards and even homes.

While voters overall across the United Kingdom opted to leave the European Union in last year's referendum, 56 percent in Northern Ireland voted to remain.

How - and whether - to keep the border invisible is among a multitude of thorny issues that need to be negotiated by the time Britain leaves the 28-country bloc in March 2019. Hundreds of laws will have to be rewritten and new trade agreements drawn up.

The E.U. and the U.K. have both said they don't want a so-called hard border. However, Ireland and the E.U. remain dissatisfied with Britain's proposals to keep the dividing line as is.

The E.U. has demanded that "significant progress" must be made on the border issue by Dec. 4 or talks about the future trade relationship won't begin as planned.

Europe currently has border controls with all countries outside the customs union or single market. The British government has said it intends to leave both, but the debate as to whether it should - or will - rages on.

To make matters worse, Northern Ireland doesn't have a government after a power-sharing agreement collapsed in January. Nationalist politicians, who want the island to be united, and unionists, who are in favor of the status quo, had worked together since the previous May.

NBC News took a road trip along the length of the U.K.'s only land border with the E.U. to examine the potential impact of Brexit.

Southwest of Londonderry, Mullenan Road passes an old customs building, since transformed into a family home.

Patrick Horner, 62, lives across the street. His garden hedge marks the border between the U.K. and Ireland, as well as the edge of his property.

"We don't need a border. Why would we?" he asked. "These are imaginary lines."

Northern Ireland was created by the 1920 Government of Ireland Act, in which Britain partitioned the island into two, with six counties in the north remaining in the U.K. A bloody civil war ensued in the south.

Nearly a century later, the question of identity weighs heavily on Horner, who says he feels Irish but officially lives in the U.K.

The Good Friday Agreement - which formally ended "the Troubles" in 1998 - afforded the people of Northern Ireland the right to identify and be accepted as British or Irish, or both, within the European Union.

Many argue this acted as the pact's glue, and that peace may be undermined without it after Brexit.

"It was our 'get out of jail free' card," Horner said. "You could be Irish, British, Pakistani or Indian and still be European ... But without that it's going to put us back into our trenches."

Peter Sheridan, the head of an all-island peace-building charity who was sitting at Horner's kitchen table, said it was the physical border that caused tensions to spill over into violence in the first place.

"Many people think the conflict was about the Catholic and Protestant religions. It wasn't. It was about identity," he said, explaining that one side of the community saw themselves tied to London and the other side to Dublin.

"When you put a border up between people and their allegiances, that's why the conflict happened," Sheridan added.

While Sheridan said he didn't think there would be a return to violence, he did say the people of Northern Ireland couldn't become complacent. "We need to learn from the past," he said.

Martina Anderson, a member of the European Parliament who is with the nationalist Sinn Fein party, agreed that Northern Ireland can't afford to jeopardize the progress it's made.

The return of a hard border would "undermine all that has been achieved in our peace process during the last 20 years," said Anderson, who spent 13 years behind bars for her role in an Irish Republican Army bombing plot before being released as part of the peace accords.

The IRA waged a three-decade campaign to win a united Ireland and is considered a terrorist group by the British government.

Anderson said a hard border would not only lead to "economic oblivion" but would "totally" undermine the peace and lead to "years of hard work undone."

Around 70 miles down the road , in Northern Ireland's southwestern-most corner, John Sheridan, the farmer, was ticking off a list of things he stands to lose after Brexit.

He fears that if tariffs are introduced for trade between the U.K. and European Union countries, the value of his lamb could drastically decrease.

"They'll put me out of business, so why wouldn't I be in a bad humor?" he said during a visit to his farm in August.

Sheridan is a member of Border Communities Against Brexit, a group of residents that came together after the Brexit referendum out of concern that Northern Ireland's vote to remain would not be respected.

Sheridan, 56, was clear that communities on either side of the border, which are now "married" and "completely interconnected," must not be ripped apart again.

He pointed to wind turbines and a ridgeline of limestone in the distance, highlighting that there was "about 15 miles of border, absolutely no fence, no anything whatsoever" as evidence of the "ridiculousness" of trying to reintroduce a barrier.

Only border communities "understand the fallacy of saying this is north, this is south," he said.

Recent talks between the E.U. and the U.K. have reassured Sheridan that an answer will be found.

Michel Barnier, the E.U.'s chief Brexit negotiator, has called for a "unique solution" to the Irish border problem but said the onus was on Britain to come forward with proposals.

A spokesman for British Prime Minister Theresa May said on Nov. 27 that Britain remained "firmly committed" to avoiding physical infrastructure at the border, but offered no new explanation of how this might work.

Gregory Campbell, a lawmaker with the Democratic Unionist Party, which is helping to prop up May's government, said the suggestion that border issues might put peace in jeopardy was "nonsense."

The British army installed roadblocks along the border to stop IRA car bombs from coming into Northern Ireland during "the Troubles."

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Boundary issues: Brexit threatens to reopen old wounds in Northern Ireland

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The status of the Irish border, which was a flashpoint during the 30-year conflict known as "The Troubles," threatens to derail Brexit talks. John Sheridan stood ankle deep in the lush grass of his farm tracing the border between Northern Ireland and the Irish Republic with his finger across the skyline.

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Film trailer

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In The Name of the Father - Trailer [www.videograbber.net]