Today, “Borderlanders” can drive along more than 200 weaving roads that cross and recross the border between the Republic of Ireland and Northern Ireland. The border stretches 310 miles between the sea inlet Carlingford Lough in the east and Lough Foyle, an estuary, in the northwest. It divides rivers, fields, and bridges. Up to 35,000 people cross the border each day.

But it has not always been this way. Before a landmark peace deal in 1998, conflict left the border militarized, with checkpoints, watchtowers, and roadblocks restricting travel. For many, dealing with the border was not just a nuisance—it was a way of life. But after the 1998 Good Friday agreement, the border became virtually nonexistent, and both sides have lived in relative peace ever since.

Now, Britain’s decision to leave the European Union (EU) threatens to disrupt the peace and bring back a “hard border” with checkpoints. Although it represents less than three percent of the United Kingdom’s population, Northern Ireland, along with the Republic of Ireland to its south, has the potential to be the region most affected by Brexit.

Post-Brexit trade and migration restrictions could complicate the movement of goods and people between Northern Ireland (part of the UK) and the Republic of Ireland (part of the EU). During the 2016 EU referendum, most Northern Irish did not vote to leave the EU: 55.8 percent voted to “Remain.” But when the UK exits the EU, England and Wales (both of which voted to Leave) will drag Northern Ireland and Scotland (both of which voted to Remain) out with it.

The island of Ireland has been divided into two countries for nearly 100 years. Unlike Southern Ireland, which in 1922 became the Irish Free State (now the Republic of Ireland), Northern Ireland continued to be part of the UK.

Northern Ireland’s history is marked by tensions between the unionist Protestant majority and a nationalist Catholic minority. Many residents are Protestant descendants of colonists from Great Britain. Hostilities culminated in “the Troubles,” a period between the late 1960s and 1998 of violent clashes between paramilitary groups. The conflict claimed an estimated 3,500 lives and resulted in tens of thousands of injuries.

“[The border] was militarized,” said Jane Morrice, former Head of the European Commission Office in Northern Ireland, in an interview with The Politic. “There were checkpoints checking people and cars…It was a mini-Berlin Wall if you like. It was the militarization which was the most difficult part.”

After decades of sporadic conflict, the 1998 Good Friday Agreement between the British and Irish governments and Northern Ireland’s political parties set out new terms for relationships between the three sides. Central to the agreement were proposals to create a Northern Ireland Assembly, to decommission weapons, to reform policing in Northern Ireland, and to release paramilitary prisoners. Today, with both countries at peace and in the EU’s single market, the border is porousat least for now.

“The day before the UK voted to leave the EU, there were reasons to be optimistic about the border,” cartographer and Queen’s University Belfast Professor Garrett Carr told The Politic. “After a lot of uncertainty and violence, the border was finally starting to work.”

In Northern Ireland, a majority of voters opted to remain in the EU. Voters living along the border preferred continued EU membership by even larger margins than their neighbors elsewhere in the country. For these “Borderlanders,” as Carr refers to those living in border areas, the Brexit vote was a brusque reminder that many in the rest of the UK consider geographically isolated Northern Ireland to be politically insignificant.

Credit: Matthew Reiner, The Politic

Now, with ongoing negotiations about borders between the EU and the UK, people on the island are not sure whether they will be able to cross into the neighboring country without checkpointsa reminder of an era that few are eager to resurrect.

“[After the Good Friday Agreement,] the border was then plain countryside, indistinguishable from any other stretch of farmland and hills in Ireland. We were free to look at the border as a living thing where things happened, rather than a site of restriction or challenge,” said Carr. “It could just be ignored. There were no checkpoints or customs there to challenge your identity as you drove home. This is a great arrangement to have arrived at. As Brexit rolls out, we must attempt to preserve it.”

Another uncertainty is whether Great Britain will leave the EU’s single market and customs union. The customs union makes the EU a unified trading area where goods can circulate freely without duties or border checks. The single market additionally permits free movement of people and servicesmost prominently, by eliminating immigration controls.

An exit from either the customs union or the single market could cause the UK to turn to non-EU countries for a greater share of its imports, a choice that would have implications for the remaining 27 EU member states. Dan O’Brien of The Institute of International and European Affairs, told The Politic that the Republic of Ireland could be influenced even more significantly. He said if Brexit has a negative impact on the British economy, it will likely have an equally negative, if not greater, effect on Ireland’s economy. At present, two-way trade between the UK and the Republic is worth 60 billion euros annually.

In particular, a customs union exit could upset UK-Republic agricultural supply chains. Milk, for example, can seamlessly cross the border multiple times (for use in pasteurization and cheese processing, for instance) before it is a finished product. If Britain exits the single market and customs union, the integrated supply chain for dairy products would be subject to customs controls, tariffs, and excise taxes.

At present, agricultural products from non-EU countries face heavy regulation. As a result, Ireland and other EU members play a large role in feeding Great Britain’s 65 million people. If Britain were to leave the single market and customs union, Irish farmers stand to lose a significant competitive advantage.

“The worst case scenario,” O’Brien said, “is that effectively, agriculture in the Republic is profoundly transformed because it loses its biggest market, which is Britain.”

The “best-case scenario” for many Irish farmers would be for Britain to remain in both the EU single market and the customs union. It is unlikely that both will happen. To stay in the single market would require free movement of people, forcing Britain to abandon the idea of immigration controls central to the “Leave” platform. And if Britain were to remain in the customs union, it would be unable to make new bilateral trade deals on agricultural products with third-party countries: another disappointment for free-market Brexiteers.

“The hope is that Britain would see that the cost of leaving the customs union would be greater than the benefits of free trade deals with third countries, and that they end up staying in the customs union,” O’Brien said. “That’s the most that can be realistically expected, certainly from an Irish perspective.”

European Union policy also requires free movement of qualified professionals who may cross internal EU borders to practice their occupation. John Woods, the Northern Ireland council chair for the British Medical Association, noted on the BBC in June that Northern Ireland relies on the Republic for certain specialist services, the quality of which may decline if Irish doctors, for example, face new border restrictions. Other cross-border health services, such as the cancer center in Londonderry, Northern Ireland that serves numerous Irish patients, could also find themselves in jeopardy.

Much of the current apprehension on the island, according to Yale political science professor Bonnie Weir, lies with the older population—people who remember the  pre-1998 Troubles and who are opposed to any sort of reimplementation of a hard border.

“[There are] thousands of people who live on one side of the border and work on the other, and their ability to get to their place of work or to their families would be severely restricted,” Weir, who has extensively studied Northern Ireland’s post-conflict transformation, told The Politic in an interview.

“That is a practical effect that would be very detrimental to the lives of so many people. But then you also have psychological effects—the number of people that still remember the heavily militarized border and the sense of a lack of control over one’s own life and where one travels—that would come flooding back immediately,” she continued.

The UK government understands this, and the future of the border has taken a prominent position in Brexit negotiations. More than a year after the Brexit vote, “Borderlanders” have few answers about their future. The Irish question is a priority in the UK’s negotiations with the EU, but so far there has been no agreement about what the border will look like post-Brexit. In late October, British Prime Minister Theresa May triumphantly assured the House of Commons that there will be no checkpoint infrastructure—in other words, no hard border of the sort that older Irish citizens remember from before 1998. It remains unclear, however, how a soft border would be possible if the UK were to exit the customs union, since the EU currently has border controls with all countries outside the customs union or single market. Many members of the British government have said the UK plans to leave both.

The UK and the EU will therefore likely have to come to a special agreement on the Ireland question. One proposal, put forth by Morrice, would give Northern Ireland honorary EU association, allowing it to remain in both the UK and the EU. She told The Politic that she had plans to present her proposal at the European Commission office in Dublin.

Across the island, local governments have mobilized in response to Brexit. In a statement released to The Politic, the Belfast City Council said that a special committee meeting has been scheduled for November to “have an informed discussion on the issue of Brexit to understand the potential impacts and to consider Belfast’s resilience to meet the challenges and take advantage of the opportunities for the future.”

Despite the challenges posed by Brexit, “Borderlanders” are sure to do everything in their power to avoid a return to their painful history.

“We have been turning around to face each other and getting to know each other,” Morrice said of the people working together to heal Northern Ireland’s wounds, “and we don’t want to turn our backs on each other.”