FLORENCECOURT, Northern Ireland-Farmer John Sheridan drives his mud-caked Land Rover up and down country lanes and roads, back and forth across the snaking border separating Northern Ireland and the Republic of Ireland. There's little sign he is crossing an international frontier.
But Sheridan remembers when this was a place of customs posts and troops. Since Britain voted to leave the European Union, he has feared Brexit could threaten the borderless life that he and his neighbors have built, putting up new barriers to trade and heightening political tensions in a region still moving on from decades of violence.
Looking out over a stretch of starkly beautiful grass and bogland where he raises sheep and cattle, 56-year-old Sheridan said that during Northern Ireland's violent "Troubles," traveling between farms meant being "stopped here, there and everywhere" by army and police patrols.
"For about a mile and a half you went through semi-permanent checkpoints of British army in the north here, southern Irish army in the south," he said.
Britain and the E.U. agreed Friday that after Brexit there must be no return to a "hard border" between Northern Ireland, which is part of the U.K., and Ireland, which will remain a member of the E.U.
How that will happen has yet to be negotiated, but Sheridan said the breakthrough gives him "a certain amount of comfort" that the border will remain open.
Outside Northern Ireland, few people in Britain were thinking about the border when the country voted in June 2016 to leave the E.U. But the 310-mile line will be the United Kingdom's only land border with an E.U. country once Britain leaves the now 28-nation bloc in March 2019.
Concern has been mounting here since the referendum about whether Brexit will mean a return to customs duties, vehicle checks and other border apparatus.
The international border severed areas that had long been intertwined. In the northwest Ireland village of Pettigo, it cut the town in half: One side of a small bridge was in Ireland, the other in the North.
Resident Mona Flood, 80, can remember when a trip across town involved border checks, documents and customs officials hunting for smuggled goods.
She showed visitors the customs booklet that her mother had to get stamped each time she drove across the border, and recalled how local people commonly hid cigarettes, alcohol, and especially butter.
"(Butter was) 2 shillings and 11 pence in the North, and you'd smuggle it in your handbag-or other places," she said. "And they'd search you.
"I hope it doesn't go back to that."
The border began to blur after the E.U.'s single market for goods, services and people was born in 1993, with both Britain and Ireland among its members.
Northern Ireland's 1998 Good Friday peace accord brought a winding-down of the Troubles. The British army withdrew, its bases and watchtowers dismantled.
The very idea of the border began to fade. Today, almost the only sign a frontier has been crossed is a switch in road signs from miles on the U.K. side to kilometers on the Irish one.
"In this community there is a sense of Brexit being so unimaginable that it is unreal," said Pettigo's local priest, Father Laurence "La" Flynn, as he paused on the bridge where the two countries invisibly meet. "We don't believe that the border will come in as a hard reality. Our minds can't get to that."
The economies of north and south have become deeply intertwined. Thousands of people cross each day to work, shop or study, and 2 million vehicles a month traverse the border carrying goods of all kinds.
Ireland's famous Guinness beer is brewed in Dublin, then shipped to Belfast to be bottled. A third of Northern Ireland's milk is sent south to be processed into butter, cheese or baby formula.
Friday's agreement says the U.K. promises to keep that trade flowing by maintaining full regulatory alignment with the E.U. on issues affecting Ireland-essentially a promise that Ireland and Northern Ireland will continue to follow roughly the same trade rules.
What that phrase means in practice will be fought over by politicians and negotiators in the months to come. The idea of "full alignment" is contentious among Brexit-backers in Britain, who worry it may tie the country's hands in trade talks with other countries.
And the Democratic Unionist Party, which is dedicated to preserving Northern Ireland's British identity, scuttled a previous version of the wording on Monday, saying it would not support a deal it saw as undermining Northern Ireland's place in the United Kingdom by treating it as a special case.
DUP politicians accused the Irish government of reckless behavior, a reminder that old divides have faded but not disappeared since the end of the Troubles, almost four decades of Catholic-Protestant violence in which 3,700 people died.
There's no immediate prospect of a return to those bad old days. But many worry that any return of the border could renew old tensions between pro-British Unionists and Irish nationalists in Northern Ireland.