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.

Watch: Dividing Lines To view this video please enable JavaScript, and consider upgrading to a web browser that supports HTML5 video Photo by Alex Bowie / Getty Images

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.