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It’s been 20 years since the Good Friday Agreement formally brought an end to a period in Northern Ireland known, perhaps too understatedly, as “The Troubles.” The three-decade conflict pitted Protestant Unionists, who wanted to preserve Northern Ireland’s status as part of the U.K., against Catholic Republicans, who sought to leave the U.K. and join the Republic of Ireland. Fighting among paramilitaries on both sides, as well as the British army, left roughly 3,600 dead during the period.

But then, on April 10, 1998, a breakthrough came. Under the Good Friday Agreement, sectarian violence on the island of Ireland has largely diminished; communal relations between Unionists and Republicans have improved; and the north-south border once hardened by military checkpoints and watchtowers has become almost invisible.

“When I announced the agreement, I described it as a historic achievement, which it was,” Senator George Mitchell, the former U.S. envoy to Northern Ireland and one of the principle architects of the Good Friday Agreement, told me. “But at the same time, I said that by itself the agreement did not guarantee peace or political stability or reconciliation. By definition, the agreement deferred major issues to the future. … It stated explicitly that the different political objectives of the two communities in Northern Ireland were both valid and anticipated that the people there would continue to advocate for their objective, but that they would not do so through violence, but rather through democratic and peaceful means.”

This goal has largely been achieved. Yet Mitchell’s warning about political stability was also prescient. A Sky News poll published Monday found that approximately half of people in Northern Ireland have few to no friends of a different religion to theirs—a metric that also stands in for political divisions. These are evident in the country’s government, or rather, its lack of one. Though one of the Good Friday Agreement’s principle achievements was the establishment of a power-sharing arrangement that allows Unionists and Republicans to govern in coalition, a political row between the Northern Irish Democratic Unionist Party (DUP) and its Republican counterpart Sinn Féin caused Northern Ireland’s government to collapse in January 2017. Belfast hasn’t had a working government since.

And then there is Brexit, which, perhaps more than any one thing, has brought into relief the tenuous foundation on which the Good Friday Agreement rests. One key to the entire arrangement was the open border between Northern Ireland and the Republic of Ireland that the European Union guaranteed. Now that once heavily militarized area is the site not only of past pain but of future uncertainty, as the only land border between the U.K. and the EU. So far, U.K. and EU negotiators have failed to agree on a solution that avoids the imposition of a hard border on the island of Ireland once the U.K. leaves the bloc.

“The DUP doesn’t want any ambiguity or doubt about the importance of Northern Ireland for the United Kingdom,” Katy Hayward, a political sociologist at Queen’s University Belfast, told me. She noted that the party’s objection to Northern Ireland remaining part of the EU customs union—a solution that has been floated by Brexit negotiators to avoid the need for a customs border—stems from political insecurities of the past. “If there’s any sense that Northern Ireland could possibly drift further away from Britain and closer to Ireland, that makes Unionists feel very anxious. … They fear that a United Ireland may come out as a result.”

The agreement never resolved the source of this fear. In fact, it explicitly avoided doing so, opting instead to create a system in which both positions could coexist peacefully. The agreement did this first by acknowledging the “continuing, and equally legitimate, political aspirations” of each side. Then came a crafty stipulation:

“...it is for the people of the island of Ireland alone, by agreement between the two parts respectively and without external impediment, to exercise their right of self-determination”

Though the agreement acknowledges Northern Ireland’s constitutional status as a part of the United Kingdom, it leaves open the possibility of a future united Ireland if majorities in Northern Ireland and the Republic of Ireland want it—a decision the accord says would be decided by referendum, and one which both the British and Irish governments would be compelled to honor.

As Mitchell had hoped, Unionists and Republicans do settle their differences through political means. And while the country’s assembly has been suspended on more than one occasion due to political deadlock, the DUP and Sinn Féin have still governed in coalition for more than a decade. The achievement this represents is truly historic, and has saved lives.

Yet the underlying sectarianism was, if anything, Hayward said, reinforced by the agreement. “The political status of Northern Ireland could change in the future, and that’s really very reassuring to Nationalists,” she said. “For Unionists, there’s a sense of a need to continually shore up that Unionist identity and not let go of that majority because of fear that if you have a move to a Nationalist identity, then you have a united Ireland. … This is why they’ve had a growing move toward voting for hardline parties, that’s why the DUP and Sinn Féin came into power—because there’s that uncertainty, there’s that sense of Northern Ireland being in limbo in a political sense.”

Brexit is now creating its own kind of limbo. Unionists are reluctant to get some kind of “special arrangement” for Northern Ireland that symbolically separates it from the rest of the U.K. in an effort to maintain close relations to the Republic of Ireland—such as, for example, keeping a customs union with the EU after the rest of the U.K. has already left it. For Republicans, the border causes similar fears of separation—albeit from the Republic of Ireland. “Having a soft border was crucial because that meant the issue of identity was really removed from the table,” Jonathan Powell, the U.K.’s chief negotiator on the Good Friday Agreement, told me. “You could live in Northern Ireland all your life and be Irish (have an Irish passport, never notice there was a border), or you could be British, or you could be both. If you have a hard border and we go back to the concrete blocks on small roads and the border point crossings and all that, then the identity issue is reopened.”

Which doesn’t mean a return to violence. “We’re not going to go back to The Troubles again,” Powell said. But he’s worried that Northern Ireland’s current leaders are less capable of compromise than their predecessors. “The cost of making compromise is no longer so life and death as it was for [Ian] Paisley and [Martin] McGuinness,” he said of the DUP and Sinn Féin leaders—affectionately known as the “Chuckle Brothers” for their amiable, albeit unlikely, friendship—who ushered in the parties’ first power-sharing arrangement in 2007. “I’m worried whether the new generation is up to this challenge, whether they can make it work.”

The U.K. and the Republic of Ireland will need to compromise, too. Though the British and Irish governments have historically held a unified stance on Northern Ireland, Brexit has put the two parties at opposite ends of the negotiating table, and Hayward said it’s starting to show. “At the moment, neither Unionists or Nationalists are preparing their constituencies for a compromise—the same thing on the British and Irish side as well,” she said, adding: “That would be my biggest worry on the impact of Brexit. We’re not prepared for it in any sense whatsoever.”

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