It is one of the stranger aspects of the Brexit debate. When the plea is raised to remember the Good Friday agreement, to do nothing that might jeopardise the fragile peace that has held in Northern Ireland for two decades, that plea usually comes in a continental European accent. Of course, Irish politicians have been saying it loudly from the start, but this week it was striking to hear French, German, Dutch or Belgian voices explaining to British TV and radio audiences why the EU couldn’t possibly accept Boris Johnson’s revised Brexit plan because of the risk it posed to peace in a corner of the United Kingdom where a bloody conflict had raged within recent memory.

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You’d think Britons wouldn’t need to be reminded. An estimated 3,500 people were killed during the 30-year war we euphemistically referred to as “the Troubles”. That violence was the background hum of life on these islands – on the news most days – until the late 1990s, in Northern Ireland obviously but also in Britain. Pubs were bombed, along with department stores and shopping centres, even a party conference. As a schoolchild, I remember how routine it became for the tube home to be closed due to “a bomb scare”.

Back then, it seemed it might go on for ever. When the various armed groups declared a ceasefire and began to talk, a resolution looked like the stuff of fantasy. Journalists whose heads ache now at the intricacies of Brexit had a foretaste with the complexities of the Northern Ireland peace process. So many circles had to be squared, so many neuralgic issues of tradition and identity had to be soothed, it demanded the most intense imaginative engagement from multiple sides: Washington, London, Dublin, unionists, nationalists, loyalists, republicans. Pulling it off was the diplomatic equivalent of a moon landing.

And yet, instead of handling this delicate creation with care, Johnson has yanked and tugged at it, breaking bits off in the process, as he attempts to squeeze it into the parcel labelled Brexit that he has vowed to deliver by 31 October. Put simply, he has raised the prospect of customs checks on the island of Ireland when the key to the Good Friday agreement was its removal of all such paraphernalia, its gentle erasure of the physical border between north and south, thereby allowing citizens of Northern Ireland to identify as British or Irish without the need for either side to accept total victory or total defeat.

Is it mere forgetfulness that explains how a UK government could handle something so precious so recklessly? Put aside the DUP, which opposed the Good Friday agreement and is ideologically committed to maintaining the distinction between the Republic and the north. It’s true that only 105 of today’s MPs were in the House of Commons when the agreement was sealed in 1998: have the rest simply forgotten how great a prize it was, and how hard it was to win it? I found myself looking up the age of Karen Bradley, Theresa May’s Northern Ireland secretary, who infamously admitted not knowing that in Northern Ireland “people who are nationalists don’t vote for unionist parties and vice versa”. Turns out she was born in 1970: bombs were going off in her 20s, just as they were in mine. Johnson is older still; he has even less excuse.

But age is not the issue. The most charitable explanation is that, after 1998, most politicians simply considered the issue done, to be filed away. But there’s more to it than that. The truth is, they were casually dismissive of Ireland because it simply didn’t matter to them. In confessing he had not read the Good Friday agreement “all the way through” the then Brexit secretary Dominic Raab was exhibiting a disdain that has proved remarkably durable.

When I called Diarmaid Ferriter, the Irish historian and author of The Border, to discuss the Johnson plan’s almost casual insouciance towards Ireland, his first words were, “We’re well used to that.” He recalled the remark of Lord Salisbury in 1924: “The average English voter has little interest in, and even less understanding of, the Irish question.” It reminded me of the time that, as a young reporter, a BBC editor had advised me that as far as the audience were concerned: “The two most boring words in the English language are ‘Northern Ireland’.”

The point is, these attitudes were expressed even when the conflict was raging. Ferriter recalls how it took great effort to engage prime minister Edward Heath on the Irish issue, even as UK citizens were being killed on UK streets. That is the context in which John Major and Tony Blair, pivotal figures in the Good Friday process, could issue their warning – that Brexit would bring back border controls in Ireland – during the 2016 campaign and yet be ignored.

As David Reynolds puts it in Island Stories, an upcoming book on British history in the age of Brexit: “During the EU referendum debate, the Conservative and Unionist party closed its eyes to recent history.” Such a blind spot is born of a great, even colonial condescension. Recall Johnson’s comparison of the Irish border to the boundary between Islington and Camden, or his fury that the issue now loomed so large: “We’re allowing the tail to wag the dog.” Johnson revealed the assumption long held by the British, and especially English, ruling class: that “little Ireland” can’t possibly get in the way of what Britain wants. Except now that it’s backed by 26 fellow members of the EU, it really can.

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Still, amnesia and imperiousness do not alone explain this carelessness towards the life-saving document that is the Good Friday agreement. It may seem a matter of apple-pie consensus now, but the peace process and eventual accord faced strident opponents – today’s arch-Brexiteers among them. The DUP was hostile throughout, of course, but so was one Michael Gove, author of a 58-page jeremiad that compared the agreement to Nazi-era appeasement, describing it as a “humiliation” and “moral stain”. Is it a surprise that this government is risking a return to violence in Ireland when it includes those who loathed the very pact that ended it?

In this fog of forgetfulness, one crucial point is especially lost. Yes, the pictures from 1998 show Bill Clinton and his handpicked US peace envoy. But it was the European Union that made the Good Friday agreement possible. It was because both the UK and Ireland were now members of a larger entity and, crucially, a shared single market, that the border between them could fade to invisibility. Tamper with that and you erode the foundation of the peace that has held for two decades.

It points to a glum truth about Brexit. Thanks to the complicated, bloody history and geography of these islands, leaving the EU, including the single market and customs union, means constructing a border between the EU and the UK in the one place where it’s just too dangerous to have such a border. Put another way, leaving the EU would be easier for every other member state than it would be for us. We should not be the first country to leave the European Union. In fact, given our past – our pain and the peace that ended it – we should be the very last.

• Jonathan Freedland is a Guardian columnist