In Philadelphia this week, a group of Irish men and women are gathered, reflecting the success of the 1998 Good Friday Agreement in bringing peace to Northern Ireland.

But they worry that Brexit — and the British parliament’s hapless efforts to exit the European Union — are already threatening Ireland’s peace. And they wish that the United States, which godfathered the Good Friday deal, would take an active role in its preservation.

They know from personal experience the ugliness that awaits Ireland if the accord fails.

They are Eisenhower Exchange fellows — part of a program that brings emerging leaders from abroad to meet their American contemporaries. They include fellows from a 1989 “Island of Ireland” group that brought Protestants and Catholics from British-ruled Northern Ireland together with citizens of the Irish Republic in the south — at a time when cross-border and inter-sectarian meetings were rare.

The 14 emerging Irish leaders, professors, politicians, civil servants and other professionals talk about the madness of rebuilding nationalist walls that the EU helped tear down.

David Lavery was a government lawyer in Belfast who had “very little contact with the Republic” before becoming a fellow. The 300-mile border between north and south was guarded by British soldiers, and watchtowers, while the IRA (Irish Republican Army) battled British soldiers and Protestant unionist paramilitaries.

After 1989, Lavery had many cross-border meetings with his Eisenhower Exchange colleagues from the republic. Ultimately those talks — with stellar Clinton administration help — produced the Good Friday Agreement, giving the north self-government and opening the border with the Irish Republic.

However, and this is key, the fact that Great Britain and Ireland were both in the European Union facilitated that open border, with no customs checks or barriers. “People started going up and down to shop for a day, bring the kids,” recalls 1989 fellow Joyce O’Connor, an education leader from the republic. “People could live on one side and work on the other.”

“The animosity was going down, road services (between north and south) increased,” recalls prominent Presbyterian minister John Dunlop, a 1989 fellow from the north.

Then came the 2016 Brexit referendum in Britain, and parliament’s paralysis. If Britain “crashes out” of the EU, while the Republic of Ireland remains in, the border between the north and south of the Irish island would be reinstated. “All the momentum we had generated, we’ve lost that in the past two years,” Dunlop says.

This prospect has already sent tensions flaring in Northern Ireland, as the leading Protestant party agitates for Brexit and Catholics oppose it. Violence has returned, and a leading young journalist, Lyra McKee, was recently shot dead in Derry in a clash between radical Catholic youths and police.

“We see the creation of fear again,” says O’Connor. “There is no certainty.”

For Dunlop, these dismal prospects make it critical that up-and-coming Irish leaders work to make their voices heard.

The restoration of borders within Ireland would be a disaster. And American leadership, so critical to producing the Good Friday Agreement, would be most welcome in saving it.

The fellows applauded U.S. House Speaker Nancy Pelosi for visiting Ireland and declaring that any Brexit deal must honor the Good Friday Agreement. President Trump, unfortunately, is a Brexit cheerleader, but perhaps a visit to Ireland could enlighten him.  Trudy Rubin is a columnist and editorial board member for the Philadelphia Inquirer.