Pedestrians walk past a mural in Derry, Northern Ireland.Jeff J. Mitchell / Getty

DUBLIN—Northern Ireland’s Good Friday Agreement, signed in Belfast 21 years ago this month, was hailed as a triumph of moderation, a hard-won compromise ending 30 years of bloodshed.

So deep were divisions, so entrenched the centuries-old feud between Catholics and Protestants, that many on the island of Ireland thought peace was impossible. Little wonder that two of the architects of the deal—John Hume, of the Social Democratic and Labour Party (SDLP), a mainly Catholic nationalist group; and David Trimble, of the mostly Protestant, conservative Ulster Unionist Party (UUP)—were jointly awarded that year’s Nobel Peace Prize. Alongside them when the deal was signed were two of their senior advisers, Mark Durkan and Dermot Nesbitt, as well as the British and Irish prime ministers, Tony Blair and Bertie Ahern. All were moderates, representatives of a new political center that not only held together, but had won a great victory.

And yet in elections only five years later, Hume, Trimble, Durkan, and Nesbitt were eclipsed in the very legislature they had created, outflanked and outvoted by groupings that appealed instead to more tribalist instincts. Their fall, in some ways, presaged shifts seen more recently elsewhere in Europe, as far-right and far-left forces have railed against outsiders, argued for communities and countries to close in, and eschewed compromise in favor of ideological purity. Rather than promoting moderation and reconciliation, the Good Friday Agreement instead pushed Northern Ireland’s voters on both sides of the sectarian divide away from the center, and toward the extremes.

The majority of Catholics in Northern Ireland, which is part of the United Kingdom, are known as “nationalists” because of their desire—in theory, at least—to see the country made part of the Republic of Ireland. Most Protestants are described as unionists because they want to retain the relationship between Northern Ireland and the rest of the U.K. From 1968 to 1998, at least 3,500 people died there in sectarian and political violence known as the Troubles. The Good Friday Agreement sought to ease tensions between the two communities by devolving power from the U.K.’s central government—introducing power-sharing between nationalists and unionists, allowing Northern Ireland’s residents to choose whether they were British or Irish nationals (or both), and all but erasing, in practical terms, the border between the two parts of the island. Now Northern Ireland is once again at the center of attention, this time not because of violence, but because of politics.

Officials in London and Brussels have puzzled over how to ensure that the United Kingdom leaves the European Union without creating a “hard” border between Northern Ireland and the Republic of Ireland, which will remain in the bloc. The issue has been at the heart of successive delays to Brexit, which may now not happen until October 31, if then. Yet in Belfast, the Northern Ireland Assembly, a body created out of the Good Friday Agreement, which should be speaking out for its people’s interests, has not held a sitting for more than two years, its two biggest parties refusing to cooperate with each other.

Durkan and Nesbitt’s journey, from architects of a landmark deal to the political margins, offers some insight into the forces at play. Durkan, the chair of the SDLP when the Good Friday Agreement was signed, served as Northern Ireland’s deputy first minister under Trimble; Nesbitt, a former economics professor and UUP negotiator, was Northern Ireland’s environment minister in the same administration. Durkan has now crossed the border, politically, to contest a Dublin seat in the forthcoming European Parliament elections. Nesbitt has retired from active politics. Both believe that their parties have fallen victim to what they see as the cynicism of two more extreme parties: Sinn Féin, the former political wing of the Irish Republican Army, and the Democratic Unionist Party (DUP), the hard-line Protestant bloc that for a time supported Theresa May’s government in London.

It is a far cry from April 10, 1998, when the Good Friday Agreement was signed. “We’d been on a roller coaster,” Durkan recalls. At the time, he says, peace “looked so far away, and then so close, and then so far away again.” So when the deal was finalized, “we knew that we’d still a mountain to climb but that we’d achieved something against the odds that day.”

But then, he says, extremist parties that had initially resisted or delayed participating in the agreement began stirring up discontent at perceived concessions made by moderates to the other side, on contentious issues such as the Irish language and the routing of traditional parades. Both Durkan and Nesbitt allege that their opponents then used that negative energy to win votes—and power and patronage—for themselves. A proportional-representation electoral system that was designed to erode old sectarian divisions in Northern Ireland failed at the task, and voters gravitated toward parties on opposite ends of the spectrum, such as Sinn Féin and the DUP, to ensure that their community had greater collective power.

“The DUP said to unionists, ‘Vote for us … or you’ll split the unionist vote,’” Nesbitt told me. “And Sinn Féin would say to nationalists, ‘Vote for us and you might not get the DUP.’” (The DUP and Sinn Féin would contest Durkan and Nesbitt’s telling, arguing that they are merely representing the concerns of their electoral bases.)

Over time, what had largely been a split contest between the SDLP and Sinn Féin on the Catholic, nationalist side, and the UUP and DUP on the Protestant, unionist side, devolved into a two-way battle between Sinn Féin and the DUP. The former now holds seven seats in the British House of Commons—though, as it refuses to acknowledge British sovereignty over Northern Ireland, it does not take its seats; the latter has 10 seats and has backed May’s minority government in London, though often voting with the hard Brexit faction in her divided Conservative Party. The SDLP and the UUP, Durkan’s and Nesbitt’s parties, are depleted forces, with no seats in Westminster and about a quarter of seats in the Northern Ireland Assembly combined, down from nearly half in 1998.

An understandable frustration exists among Northern Ireland’s moderate unionists and nationalists at seeing their hard-won institutions taken over, and ultimately paralyzed, by hard-liners who questioned or opposed their creation.

The result, however, has been that by 2017, Northern Ireland politics were again so polarized on sectarian lines that the cross-community cooperation at the heart of the Good Friday Agreement could no longer function. The immediate causes of the collapse in power-sharing were unionist opposition to legal status for the indigenous Irish language and the refusal of the then–first minister, Arlene Foster of the DUP, to resign over a fuel-subsidy scandal. The DUP–Sinn Féin coalition, always a cold marriage of convenience, broke down after 10 years of cohabitation.

On a day-to-day basis, there has been surprisingly little disquiet or unrest in Northern Ireland. Residents still go about their business, and the civil service continues to function. People in Northern Ireland have always known and accepted that they are deeply divided, and seem to have learned to cope with it.

Yet the long-term risks are real. For one, as the United Kingdom wrestles with the question of Brexit, Belfast has had no voice of its own in the process. And dissident paramilitaries remain active, though marginalized, in both the Catholic and Protestant communities. There are fears that a new hard border would be a target for Irish republican militants seeking a return to what they see as the “armed struggle” against British sovereignty. The killing of a journalist last week in Derry, in Northern Ireland, illustrates the fragility of the peace. The decision of DUP lawmakers in Westminster to buck mainstream Northern Ireland opinion—which favored remaining in the EU by a 56–44 margin in the 2016 Brexit referendum—and vote for a clean break with the EU has pushed some moderate nationalists to reconsider their support for the Good Friday Agreement. Talk of a united Ireland, largely hushed by the settlement, is being heard again, even among moderate nationalists.

Time would indeed appear to be on the side of the Irish nationalists. Northern Ireland was carved away from the majority-Catholic island of Ireland in 1921 to create a new statelet with a two-to-one Protestant majority. But the most recent census, carried out eight years ago, showed that people raised as Catholics made up 45 percent of the population. With a younger demographic profile, they will likely soon be in the majority. And the Good Friday Agreement makes specific provisions for a poll to end partition, with a simple majority on both sides of the border enough to unite the island.

At present, this seems a remote possibility. A recent Irish Times/Ipsos MRBI poll suggests that only 38 percent of people in Northern Ireland want such a vote, and even less than that would vote in favor of a united Ireland—though even debating the issue has the potential to cause instability. But the erosion of faith in the political settlement, the threat of a new hard border with the Republic of Ireland, and the Brexit-fueled tensions between the chief external guarantors of the deal, Dublin and London, mean that peace can no longer be taken for granted.

Yet neither Durkan nor Nesbitt despairs for the future. Opinion polls still suggest a great deal of common ground between voters on issues such as maintaining a soft border, preserving EU trade links, and ending a ban on abortion. Despite their reputation for hard-line positions, both Sinn Féin and the DUP have had to greatly moderate their stances since the Good Friday Agreement, committing themselves more than they would originally have liked to power-sharing institutions and, in Sinn Féin’s case, grudgingly acknowledging British sovereignty over Northern Ireland.

“The dynamic that we tried to set in motion,” Nesbitt said, “is still there.”

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