LONDON - From Lough Foyle to Carlingford Lough, the border separating the Republic of Ireland and Northern Ireland snakes for about 500 kilometers through peaceful countryside.
Most of the time nowadays, travelers only know they have crossed from one side to the other of the partitioned island because of the difference in the color of the mailboxes or because road signs in the south use kilometers, while in the north they're indicated in miles.
Border posts and checkpoints are a thing of a troubled past.
But that might change, if Britain leaves the European Union without an exit deal. The quiet, bucolic border separating the island of Ireland could well be both the victim and the cause of a sharp rupture between London and Brussels that would impact jobs and disrupt businesses on both sides of the English Channel.
European Council President Donald Tusk warned Monday that Britain is barreling toward a 'no-deal Brexit,' which, aside from hardline Brexiters in British Prime Minister Theresa May's ruling Conservative party, no one on either side of the English Channel wants. A no-deal Brexit is 'more likely than ever before," Tusk said just hours ahead of a summit of European leaders in Brussels that EU officials have billed as the "moment of truth" for Brexit.
Britain's May will face EU leaders Wednesday to make what some believe could well amount to a final bid to persuade them to agree on more concessions and to rescue Brexit talks that have ground to a halt over how to avoid 'hardening' the border in Ireland, which would involve the imposition of customs and immigration checks.
FILE - British Prime Minister Theresa May prepares to address a press conference at an EU summit in Brussels, Belgium, June 23, 2017.
Britain agreed to remove checkpoints along the border in the 1998 Good Friday Agreement - a deal struck among London, Dublin and most political parties and armed factions in Northern Ireland that brought peace to Northern Ireland after decades of deadly violence known locally as 'the Troubles.'
Hopes of a breakthrough had been raised last week. British and EU officials thought they were close to an agreement about the Irish border, but on Sunday that all fell apart when it became clear Prime Minister May didn't have the full support of her Cabinet and that several key ministers were on the brink of resigning. Britain's Brexit minister, Dominic Raab, made an unscheduled trip to Brussels to say the deal, known as the backstop, an EU backup plan to avoid a hard Irish border, was off.
The EU wants Northern Ireland, regardless of whether there's a subsequent trade deal with Britain, to remain inside the bloc's customs union so as to avoid the necessity of border controls between the British province and the Irish Republic. Brussels is backed in this demand by the Irish government, which wants Northern Ireland to remain closely tied to European regulations on goods and services, including customs arrangements, after Britain leaves the bloc.
May says treating Northern Ireland differently from the rest of Britain would mean, in effect, that there would have to be "a border in the Irish Sea," separating two parts of Britain.
The EU proposal on how to handle the Northern Ireland border "threatens the integrity of our United Kingdom," she told British lawmakers Monday in bruising exchanges in the House of Commons. But Ireland's foreign minister, Simon Coveney, says a backstop is essential.
FILE - Activists with the group 'Borders Against Brexit' protest in the border town of Dundalk, Ireland, June 28, 2018.
"No one wants to trigger the backstop, but it needs to be there as an insurance mechanism to calm nerves that we are not going to see physically a border infrastructure emerging on the island of Ireland," he said.
Britain has counter-proposed a different 'backstop' instead, which would keep all of Britain in a customs union with the bloc temporarily, while a trade deal is negotiated with the EU. May told British lawmakers she believes an overall deal with the EU down the road is still achievable so that keeping Britain in a customs union to avoid a 'hard' Irish border is merely a defensive position that would never actually be used.
Complicating the position for the British prime minister - and the EU - is that she heads a minority government and has to contend with a large, rebellious Conservative faction, led by former Foreign Secretary Boris Johnson and aristocratic lawmaker Jacob Rees-Mogg, which wants Britain to break sharply from the EU. At least three members of May's Cabinet are aligned with the rebels.
The rebel faction, which possibly numbers more than 60 Conservative lawmakers, opposes Britain remaining for any length of time in a customs union with the EU as that would limit Britain's power to strike trade deals around the world. The faction also fears a temporary customs union membership would turn into a permanent arrangement.
Even if a border deal with the EU is struck, the chances are high that May won't get parliamentary backing for it, because of opposition from the Conservative rebels and from Northern Ireland's Democratic Unionist party, or DUP, whose 10 lawmakers in the House of Commons are needed by May to get legislation passed.
The DUP, a right-wing party opposed to reunification with the Republic of Ireland, says it won't accept any deal that treats Northern Ireland differently from the rest of Britain. The party says it's all a trick to thwart Britain exiting the EU. The party's leader in the House of Commons, Nigel Dodds, says Britain should leave the EU 'together with no part hived off either in the single market or customs union differences."
The overall head of the DUP, Arlene Foster, says she's ready to scupper any deal she dislikes by collapsing May's government. She believes Britain is now likely to leave without a deal and had a "difficult and hostile" exchange with the EU's chief negotiator, Michel Barnier, last week.
"I don't see how she can get the votes to get this through. I think we have reached a breaking point," said Hugh Bennett, news editor at Guido Fawkes, a Conservative blog site.
Meanwhile, positions are hardening among British lawmakers who either don't want Britain to leave the EU or want to maintain close ties with Europe. More of them are supporting a call for another referendum on EU membership, on the grounds that parliament, as well as the ruling party, are hopelessly divided.