Northern Ireland is no longer riven by sectarian violence, but its deeply divided politics have thrown Britain's Brexit plans into disarray.
Prime Minister Theresa May's government was holding talks Tuesday with its Northern Irish political ally in a bid to salvage an agreement with the European Union ahead of a deadline next week.
Britain and the EU came close Monday to agreeing on key divorce terms, including how to maintain an open Irish border after the United Kingdom – including Northern Ireland – leaves the EU. But the agreement was scuttled at the last minute when the Democratic Unionist Party (DUP) – which props up Ms. May's minority government – warned it would not support a deal it saw as undermining Northern Ireland's place in the UK.
The party's leader in Parliament, Nigel Dodds, said the proposed deal was "clearly unacceptable in its current form."
May characterized the problem as a minor hiccup, saying negotiations with the EU had made progress but "there are still a couple of issues we need to work on." She said talks would reconvene by the end of the week.
Britain and the EU have only days to make a deal before a Dec. 14-15 EU summit that will decide whether Brexit talks can move on to future relations and trade. The lack of progress so far has raised concerns that Britain may not have a deal by the time it officially leaves on March 29, 2019.
EU Commission spokesman Margaritis Schinas said the bloc would restart talks as soon as Britain was ready.
"The show is now in London," he said.
The crisis is a sharp reminder of the fraught and delicate politics of Northern Ireland, which has left behind decades of Catholic-Protestant violence but not the conflicting allegiances and identities that drove the "Troubles." Almost 3,700 people were killed in nearly four decades of violence.
After Britain leaves the bloc, the currently invisible 310-mile frontier between Northern Ireland and Ireland will be the UK's only land border with the EU. A generation ago, the border was marked by military watchtowers and checkpoints, and many crossings were blocked to stop the movement of militants and smuggled goods.
Customs controls were abolished when the EU single market was established in 1993, and security checks began to disappear after the 1998 Good Friday Agreement peace accord. Today the border has no customs posts or other infrastructure, and thousands of people live on one side and work, shop or go to school on the other.
"The last 20 years have nearly erased it out of our mentality," said John Sheridan, whose Northern Ireland farm abuts the border with the south.
Like many, Sheridan worries that any hardening of the border could undermine that progress and renew old tensions between pro-British Unionists and Irish nationalists. Sheridan also sends his cattle and sheep cross the border, and says tariffs would devastate his business.
Britain says it wants to maintain a "frictionless" flow of people and goods with no border posts after Brexit. But Ireland and the other EU nations are demanding to know how that will work if Britain is outside the EU's borderless single market and its tariff-free customs union, a looser trading bloc that includes non-EU states like Turkey.
Negotiators were discussing an agreement that would commit Britain to maintaining "regulatory alignment" between Northern Ireland and Ireland after Brexit in order to keep the border transparent for trade, without customs posts or other obstacles.
That language alarmed the DUP, a Protestant Unionist party that opposes any special deal to keep Northern Ireland's economy closely aligned with the Republic of Ireland.
The DUP's Mr. Dodds said Dublin politicians "are flexing their muscles ... in a reckless and dangerous way that is putting at risk years of good Anglo-Irish relations."
The DUP has only 10 seats in Britain's Parliament but May relies on its support to stay in power. If she ignores the party's wishes her government could fall, triggering a new election with the opposition Labour Party ahead in the polls.
The party's intransigence has been seized on by British supporters of "soft Brexit," who want to maintain a close relationship with the EU. Some suggested that if Northern Ireland was given special status, Scotland, Wales, or even London could seek the same. They suggested that the solution was to keep all of the UK inside the single market and customs union.
The government has ruled that out, but Labour Party Brexit spokesman Keir Starmer said it should reconsider.
"If the price of the prime minister's approach is the break-up of the Union and reopening of bitter divides in Northern Ireland, then the price is too high," Mr. Starmer said.
Starmer said the Conservative government's fantasies about Brexit had collided with "brutal reality."
"Yesterday the rubber hit the road," he said, adding: "The DUP tail is wagging the Tory dog."
This story was reported by The Associated Press.