Even the most able media performers felt that “they simply lacked real knowledge of the issue,” the ex-staffer, Oliver Norgrove, recently wrote in a column in the Irish Times. What’s more, the fate of the border seemed like a minor question: “I remember quite vividly the feeling of unease and discomfort about the prospect of us talking about something we just didn’t feel needed addressing,” Norgrove wrote.

In a classic case of the return of the repressed, the question the supporters of Brexit refused to address has come back to haunt them. Theirs was a dream of a simple, once-and-for-all escape from the past 46 years of history. Britain would erase the recent past, in which its destiny has been intertwined with that of its continental neighbors, and begin a new and glorious story. But there is another history, one in which Britain has been entwined, for many centuries, with an even closer neighbor, Ireland. That story cannot be erased. And the impossibility of escaping from it has, in the end, made Brexit itself impossible.

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If Brexit has come crashing down to earth, the piece of Earth in question is a straggling, meandering, perplexing and porous line on the map: the 310-mile border between Northern Ireland and the Republic of Ireland. It emerged as a temporary line of partition in 1920 — ironically during another episode in which a country was exiting a larger union. Catholic Ireland was breaking away from the United Kingdom. The Protestant-dominated Northeast wanted to remain. So it was agreed that there would be a short-term boundary until a permanent solution was found.

That never happened. The temporary line became a permanent, fraught border. Irish nationalists regarded it with resentment as an improper imposition on the natural unity of the island. Unionists, on the other hand, regarded the border as their defense against being absorbed into a Catholic-dominated United Ireland against their will. During the 30-year conflict that racked Northern Ireland — between 1968 and 1998 — the border became one of the most heavily policed in the world, with armed troops, watchtowers and the buzz of military helicopters overhead. For the communities separated by it, it was a daily reminder of bitter and violent difference.

And then a wonderful thing happened. Britain, Ireland, the E.U. and the United States worked together to create one of the finest diplomatic achievements of the past 50 years: the Belfast Agreement of 1998 (also known as the Good Friday Agreement). With peace, the border more or less vanished. The military and police installations were removed, and because both Ireland and Britain were in the E.U., there was no need for customs posts, either. For 20 years, people have come and gone freely over about 300 crossings to work, trade and socialize — 105 million times a year. When I travel now from Dublin to Belfast, I struggle to remember where the border is — it is a line on a map, not a barrier on the road. The most the traveler is likely to encounter is a sign saying “Welcome to Northern Ireland." It would be hard to overstate how much this has contributed to the sense of normality and the building of ordinary human contact that underpin the hard-won peace. A generation has grown up with this physical and psychological freedom.

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But along came Brexit. Its effects, if implemented in their pure form, would be not just to restore a hard border between Northern Ireland and the Irish Republic but also to make it much more extreme than it ever was before. It would now be a major E.U. land border, a boundary between Britain on the one side and a 27-member bloc on the other. This would be profoundly unsettling for a peace process that has made huge progress but is still fragile.

The naked truth is that the prospect of disrupting the peace agreement really does not matter to the voters in Britain who backed Brexit. Asked whether “the unraveling of the peace process in Northern Ireland” is a “price worth paying” for a Brexit that allows them to “take back control,” fully 83 percent of “Leave” voters agree that it is. This attitude reflects the recklessness of those who campaigned for Brexit, their blithe ignorance of and indifference to the consequences of what they were proposing. In this, the border question merely points to the broader reasons the whole Brexit project is such a dismal failure: It was never more than a set of slogans. It was all about the joy of exiting, with no clear sense of what condition the United Kingdom might be exiting into.

Brexit is in part a product of a resurgent English nationalism, and polls have shown that those who voted Leave are not too concerned if it leads to the breakup of the United Kingdom. In one recent survey, over 60 per cent of Leave voters said they would not be greatly or at all concerned were Northern Ireland to depart from the U.K. and join a United Ireland. Since the violence of the Troubles diminished, this part of the British state has been increasingly out of sight and out of mind.

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While Britain, which is the sovereign power in Northern Ireland, did not take its needs seriously, the E.U. did. For all its faults, the E.U. does understand itself as a peace project. What happens in Northern Ireland matters to its sense of Europe’s identity. So the E.U. has fully backed Ireland in its insistence that there can be no hard border after Brexit. And this means that Northern Ireland (and, hence, the United Kingdom as a whole) has to remain aligned with the E.U.’s customs union and single market “unless and until” some other way of keeping the border invisible can be found. This “backstop” is written into the withdrawal agreement and has been, for the British, its most contentious aspect. It kills the fantasy of a clean break from recent history and makes Brexit a largely pointless exercise.

In theory, the U.K. could just leave the E.U. without a deal and ignore the consequences for Ireland. But that option is so damaging to its own economy that only a small rump of fanatics still clings to it. The alternatives to the backstop put forward by the supporters of Brexit have failed to gain any purchase. One was a fabulous deal with the E.U. in which “frictionless” trade will continue – in other words the U.K. leaves the E.U.’s club but still enjoys all the benefits of being in. It is a fantasy. The other was the idea that technology could be used to monitor cross-border trade with no physical infrastructure. Such technology does not yet exist. So if we exclude make-believe and science fiction, we are left with the unpalatable reality that a post-Brexit U.K. has to stay so close to the E.U. that leaving hardly seems worth the bother.