The United Kingdom may be an island nation, but it has one, huge open border with the European Union. And closing it could cause very serious problems.
The Republic of Ireland, which is an EU member state, and Northern Ireland, which is part of the U.K., haven't been united for almost 100 years. In the past two decades though, the two have become intertwined in trade and culture.
The U.K.'s exit from the EU could change this dramatically. New border controls threaten to undermine two decades of peace between former sectarian rivals, and dampen economies in both Irish regions.
"There is fear in the north and in the south of Ireland that this is a step toward opening up an old wound that was healing wonderfully," said Peter Moloney, a visiting professor at Boston College, who studies the evolution of modern international governance.
The current border runs more than 300 miles east to west, almost invisible between farms and rolling hills.
Lines were stricter in the 1970s, as violence between Protestants and Catholics known as "Troubles" escalated. But even then, it was never an immigration border, Moloney said. It dealt primarily with custom and tariffs until 1973. The type of border that emerges post-Brexit will make a notable difference, he said.
"The danger is if you bring back checkpoints, if you bring back military towers that have been diffused for 20 years, you're bringing back nightmares from the past that people thought they left behind," Moloney said.
A main pillar of the "leave" campaign in the U.K. was claims that it is necessary to control immigrant flows into the country. But that would require the very type of borders that many Irish fear. The line between Ireland and Northern Ireland will now be the only land border between the United Kingdom and another EU country. If campaign promises are kept, it's about to become much less porous.