If the ever-stalling Brexit negotiations range in character from bewildering to boring, there was at least something familiar about them this week. To wit, the idea that a border wall could be coming, and that someone else was going to pay for it.

It wasn’t President Trump offering his take on the U.K.’s impending exit from the EU. Rather, it was British lawmaker Kate Hoey, stating her own vision for what should happen if a hard border returned between Ireland (part of the EU) and Northern Ireland (part of the U.K.). A vocal Brexit supporter, the Labour party lawmaker argued Monday that anything resembling a physical barrier on what will become the only land border between the U.K. and the EU was unnecessary, and that if the EU wanted one, it would have to put it up alone. “We’re not the ones who’re going to be putting up the physical border,” Hoey told BBC Radio 4. “If this ends up with a no deal, we won’t be putting up the border—they’ll have to pay for it because it doesn’t need to happen.”

Hoey’s comments came in response to the Irish government’s recent insistence that it would not assent to Brexit talks moving on to phase two of the negotiations, unless the U.K. government guaranteed in writing that no hard border would return between Ireland and Northern Ireland. Such a border, which could involve customs checks and controls, would significantly change the boundary that exists today, namely by rendering it visible. Currently, it’s only by noticing subtle changes in the color of road signs or the shift in speed limit metrics that a traveler can tell where one country ends and the other begins.

A hard border would change that. And while Hoey’s opposition to such a change is shared on both sides of the Brexit negotiating table, her proposal that they therefore should simply not impose one is not—in part because it fails to recognize the circumstances that could make a stronger border necessary in the first place. In addition to leaving the EU, the U.K. has also opted to leave the bloc’s single market and customs union—membership to each of which has allowed goods and services to travel between the U.K. and the rest of its member states tariff-free. Absent a trade deal (which could take years to negotiate) or some sort of transition period, the U.K. will be subject to tariffs like any other country outside the single market and customs union once it leaves the EU in March 2019. This would make at least a customs border on the island of Ireland necessary from that point forward.

It’s not just EU law that mandates this. As a member of the World Trade Organization (WTO), whose rules the U.K. will fall under once it leaves the EU, the U.K. will be required to impose its own customs checks. “The idea that you can just not have a border with the EU is mind-bogglingly ill informed,” Edward Burke, an assistant professor in international relations at the University of Nottingham, told me. “The WTO is really unyielding on this point that for international trade to work, every state needs to essentially police its borders from a customs point of view. … It’s non-negotiable.”

Both the U.K. and the EU have stressed the importance of finding a “flexible and imaginative,” solution to the Irish border issue, with some proposing that Northern Ireland be permitted to independently remain within the bloc’s single market and customs union—a special status that would effectively resolve the U.K.-EU land border issue and leave trade and freedom of movement within the island of Ireland largely unaffected. But it’s a solution that many hardline Brexiteers, including the Northern Irish Democratic Unionist Party (DUP), have rejected.

“They simply will not entertain at this point any potential exceptions, as complicated or as difficult but ultimately economically beneficial as they are,” Burke said of the DUP, which is currently propping up U.K. Prime Minister Theresa May’s Conservative government in London. He noted that the party’s opposition to this is more political than economic, stemming from “visceral fears that go back to the idea that once you detach Northern Ireland in a more significant way from the rest of the U.K. in how it conducts international trade agreements [and] how it relates to the EU, that it’s essentially a political backdoor then for future conversations about Irish unity.”

The reason for that largely has to do with Irish and Northern Irish history, dating back to the decades-long period of sectarian fighting from the 1960s through much of the 1990s known as the Troubles. During this period, Unionists (who are represented by parties like the DUP) advocated for Northern Ireland to remain part of the U.K., while Republicans (who are now largely represented by parties like DUP’s opposition, Sinn Féin) advocated for the North to join the Republic of Ireland. While the conflict ended with the signing of the Good Friday Agreement in 1998, the underlying sensitivities are still in place. And on the border, it was the EU that mitigated them; Northern Ireland remained part of the U.K., satisfying Unionists, while being deeply integrated with the Republic of Ireland, as the Republicans wanted.

Katy Hayward, a political sociologist at Queen’s University Belfast, told me the effect is already visible in communities along the border, noting in her research that Brexit’s impact has already been “polarizing” for those living on either side. “It’s still a very important dividing line distinguishing between people’s political preferences,” Hayward told me in October, adding that “the legacy of conflict and the legacy of complex British-Irish relations—they’re all intertwined in the border.”

But it’s Northern Ireland that stands to be the most vulnerable if a hard border is reimposed. While much of the country’s commerce relies on the U.K., a significant amount also relies on the Republic of Ireland, which accounts for 34 percent of its trade. More than half of its exports go to the EU overall. “The economics point in one direction, and the politics point in the opposite direction,” Professor Richard G. Whitman, an associate fellow with the Europe Programme at Chatham House, told me. “The problem for the DUP is they’re sort of doing the splits on that, because obviously anything that leads to a significant downturn in Northern Ireland’s economy is obviously going to hit their supporters and potentially hit them at the ballot box. On the other hand, you’re butting up against core Unionist principles in terms of the fact that Northern Ireland should not be in any way different from the mainland.”

Northern Ireland’s precarious political situation hasn’t helped, either. Though the DUP currently enjoys a powerful role within the U.K. government, Northern Ireland hasn’t had a government in nearly a year. Since the power-sharing government between the DUP and Sinn Féin collapsed in January, Belfast has been relegated to being a political observer to Brexit negotiations and, by extension, its fate outside the EU. “The interests of Northern Ireland have been outsourced to Dublin and to London,” Whitman said. “They are in a way bystanders at the moment to what Dublin and to what London are suggesting would be in the best interest of Northern Ireland, and that’s probably not a good thing for Northern Ireland.”

“The U.K. is going off on its own trajectory and its economic interests—the economic interest of London—are not the same as those of Northern Ireland in many ways,” Hayward said, adding: “If Ireland is turning its face away from Britain and trying to secure its own interests, then where does this leave Northern Ireland? It almost feels like sort of falling in between the cracks—between Britain and the Republic of Ireland. So this is a deep concern.”

With negotiations progressing on the main sticking points about the separation, the Irish border remains the most uncertain issue hanging over negotiations. Whether talks will move on to the next phase of negotiations will depend on what EU leaders decide when they convene for the European Council meeting next month—a decision that will no doubt be informed by Dublin. “If the U.K. offer is unacceptable for Ireland, it will also be unacceptable for the EU,” European Council President Donald Tusk said Friday. “This is why the key to the U.K.’s future lies—in some ways—in Dublin, at least as long as Brexit negotiations continue.”