Former President Bill Clinton, from left, former U.S. Sen. George Mitchell and former Secretary of State Hillary Clinton chat Monday on the first day of a three-day international conference at Queen’s University Belfast to mark the 25th anniversary of the Good Friday Agreement, in Belfast, Northern Ireland. Christophe Ena/Associated Press

BELFAST, Northern Ireland — George Mitchell, an American architect of Northern Ireland’s historic 1998 peace accord, on Monday urged its feuding politicians to revive the mothballed Belfast government, as a current political crisis clouded the celebration of the peacemaking milestone.

A former U.S. senator from Maine who served as majority leader, Mitchell told a conference to mark a quarter century since the Good Friday Agreement that Northern Ireland’s leaders must “act with courage and vision as their predecessors did 25 years ago,” when bitter enemies forged an unlikely peace.

Mitchell, the U.S. special envoy who chaired two arduous years of negotiations that led to the accord, joined ex-President Bill Clinton and political leaders from the U.K., Ireland, and Northern Ireland at Queen’s University Belfast to mark 25 years since the agreement largely ended three decades of sectarian bloodshed – a moment, Mitchell said, “when history opened itself to hope.”

“The people of Northern Ireland continue to wrestle with their doubts, their differences, their disagreements,” said Mitchell, who is now 89 and being treated for leukemia. But, he added: “The people of Northern Ireland don’t want to return to violence – not now and not ever.”

“The war is over,” agreed Gerry Adams, former leader of Sinn Fein, the party linked during the conflict to the Irish Republican Army, which killed around 1,800 people. “The conflict’s finished.”

Britain’s former Prime Minister Tony Blair, right, and former U.S. Sen. George Mitchell smile during the bust unveiling Monday at Queen’s University Belfast. Peter Morrison/Associated Press

The Good Friday Agreement has been held up around the world as proof that bitter enemies can make peace. It committed armed groups to stop fighting and set up a Northern Ireland legislature and government with power shared between unionist and nationalist parties.

Northern Ireland has changed dramatically since then. A young peacetime generation is increasingly shedding the rival identities – British Unionist and Irish nationalist – that erupted into three decades of bloodshed that killed 3,600 people. But at the same time, Northern Ireland is locked in a political crisis that threatens to rattle the peace secured by the Good Friday Agreement. And violence hasn’t disappeared completely. In February, IRA dissidents opposed the peace process shot and wounded a senior police officer.

“You’ve got a transformed society in which (the labels) unionist, nationalist for many young people doesn’t mean anything,” said Katy Hayward, professor of political sociology at Queen’s University Belfast, the conference venue. “But on the other hand, society is in a state of quite severe disrepair. We haven’t had a functioning Assembly for four out of the last six years, and our public services are crumbling around our ears.”

Increasing numbers of people wonder whether the accord that created peace is still capable of sustaining it. Northern Ireland’s 1.9 million people have been without a functioning government since the main unionist party walked out more than a year ago to protest post-Brexit trade rules that – like so much in Northern Ireland – roiled notions of history and identity.

Former U.S. Sen. George Mitchell, left, and former Secretary of State Hillary Clinton chat at Queen’s University Belfast on Monday. Peter Morrison/Associated Press

Participants at the conference – gently or pointedly – urged the Democratic Unionist Party to return to the power-sharing government.

Former U.S. Secretary of State Hillary Clinton, Queen’s University’s chancellor, urged people in Northern Ireland to show the same “unstoppable grit and resolve” that secured the peace deal.

“You have always found a way through, and I believe you will again,” she told delegates.

Sinn Fein’s Adams predicted the political impasse “will be resolved” by the DUP returning to government.

“As ministers, they have the mandate to do that,” he told The Associated Press. “We can disagree on all of these other matters, but we should do it based on the political and institutional office that we are entitled to on behalf of the people who elected us.”

The three-day conference caps commemorations of April 10, 1998, peace accord that included a flying visit last week by President Biden, on his way to explore his Irish roots in the neighboring Republic of Ireland. During speeches in Belfast and Dublin, Biden reminded Northern Ireland’s politicians how strongly the U.S. remains invested in peace.

“I wanted to make clear there’s a lot at stake, a lot at stake,” Biden told reporters as he left Ireland on Friday.

Former U.S. Sen. George Mitchell looks at a bust of his likeness during its unveiling Monday, the first day of a three-day international conference at Queen’s University Belfast to mark the 25th anniversary of the Good Friday Agreement, in Belfast, Northern Ireland. Christophe Ena/Associated Press

U.K. Prime Minister Rishi Sunak, who is due to host a gala commemorative dinner in Belfast on Wednesday, hailed “the courage, imagination, and perseverance” of the peacemakers.

But critics say the U.K. government has been, at best, careless with Northern Ireland’s peace — especially by leading Britain out of the European Union following a 2016 referendum.

“Brexit was a disaster for the peace process,” said Bertie Ahern, who was Ireland’s prime minister during the 1990s peace talks. “It opened up things that were closed.”

Brexit destabilized the delicate political balance in Northern Ireland, by reviving the need for a customs border between the EU and now ex-member the U.K. An open border between Northern Ireland and EU member Ireland is one of the foundations of peace, so checks were imposed instead on goods moving from mainland Britain to Northern Ireland.

That unsettled unionists, who see the economic barrier as undermining Northern Ireland’s place in the United Kingdom. The DUP walked out and has not returned, despite a deal reached by the U.K. and the EU in February to remove many of the border checks.

Increasing numbers of people argue that power-sharing must be tweaked to reflect the growing importance of forces such as the Alliance Party, which defines itself as neither unionist nor nationalist.

Former U.S. Secretary of State Hillary Clinton, third right, former U.S. President Bill Clinton and Britain’s former Prime Minister Tony Blair, right, look at former U.S. Sen. George Mitchell during his bust unveiling Monday. Peter Morrison/Associated Press

DUP lawmaker Ian Paisley Jr. warned that changing the terms of the peace accord risked “unraveling” the whole agreement.

Ahern said that despite the problems, the Good Friday Agreement was “a huge achievement.”

“I think so far, so good, and then we have to just try and – as George Mitchell said – do better.”

Blair urged Northern Ireland to do “the right thing.”

“We know the peace isn’t perfect,” he said. “We know the institutions have often been rocky and unstable as they are today. We know there’s still a lot of distrust and mistrust between the communities. But we also know that Northern Ireland is a much better place than it was before the Good Friday Agreement.”