The rather patronising English joke used to be that whenever the Irish question was about to be solved, the Irish would change the question. And now, when the Irish question seemed indeed to have been solved, at least for a generation, it is the English who have changed the question.

Recklessly, casually, with barely a thought, English nationalists have planted a bomb under the settlement that brought peace to Northern Ireland and close cordiality to relations between Britain and Ireland. To do this seriously and soberly would have been bad. To do it so carelessly, with nothing more than a pat on the head and a reassurance that everything will be all right, is frankly insulting.

Just five years ago, when Queen Elizabeth became the first reigning British monarch to visit southern Ireland in a century, there was a massive sense of relief. It was not just relief that the visit went off peacefully and well. It was much deeper than that: it was relief from centuries of both British condescension and Irish Anglophobia. A long story – often nasty, sometimes merely tediously wasteful – was over. There was a dignified, decent, democratic settlement that allowed the natural warmth of a neighbourly relationship to come fully to the surface.

I never imagined then that I would ever feel bitter about England again. But I do feel bitter now, because England has done a very bad day’s work for Ireland. It is dragging Irish history along in its triumphal wake, like tin cans tied to a wedding car.

All but a few diehards had learned to live with the partition of the island of Ireland. Why? Because the border between Northern Ireland and the Republic had become so soft as to be barely noticeable. If you crossed it, you had to change currencies, and if you were driving you had to remember that the speed limits were changing from kilometres per hour to miles. But these are just banal details. They do not impinge on the simple, ordinary experience of people sharing an island without having to be deeply conscious of division.

A ‘money exchanged’ sign at the border between Newry in Northern Ireland and Dundalk in the Republic of Ireland. Photograph: Paul Faith/AFP/Getty Images

What will now happen is not that the old border will come back. It’s much worse than that. The old border marked the line between neighbouring polities that had a common travel area and an intimate, if often fraught, relationship. It was a customs barrier. The new border will be the most westerly land frontier of a vast entity of more than 400 million people, and it will be an immigration (as well as a customs) barrier.

It will, if the Brexiters’ demands to take back control of immigration to the UK are meant seriously, have to be heavily policed to keep EU migrants who have lawfully entered the Republic from moving into the UK. And it will run between Newry and Dundalk, between Letterkenny and Derry. The Dublin-Belfast train will have to stop for passport controls. (Given that the border could not be secured with army watchtowers during the Troubles, it is not at all clear how this policing operation will work.)

Meanwhile, the cornerstone of the peace settlement, the Belfast agreement of 1998, is being undermined. One of the key provisions of the agreement is that anyone born in Northern Ireland has the right to be a citizen of the UK or Ireland or both. What does that mean in the new dispensation? Can someone be both an EU citizen and not an EU citizen? Likewise, the agreement underpins human rights through the “complete incorporation into Northern Ireland law of the European Convention on Human Rights”. Though not strictly required by Brexit, the leave leadership is committed to removing the convention from UK law – in other words to ripping out a core part of the peace settlement.

But the Belfast agreement isn’t some minor memorandum. It is an international treaty, registered with the United Nations. It is also arguably the greatest modern achievement of British diplomacy, partly crafted by public servants and made possible by British politicians, especially John Major and Tony Blair. It is one of the most successful models for conflict resolution around the world. Messing around with it is an insult, not just to Ireland, but to Britain’s international standing.

This fecklessness in turn is deeply unsettling for unionists in Northern Ireland. It suggests that the new English nationalism is completely indifferent to their fate. During the referendum debates, a few pro-remain voices, such as the TUC general secretary, Frances O’Grady (herself of Irish descent), tried to make a gentle plea to voters to think about Ireland and the Belfast agreement. They went unheard. English nationalists, it turns out, wouldn’t give the froth off a pint of real ale for the Irish peace process.

And if they don’t care enough even to talk in any serious way about the consequences of Brexit for Northern Ireland, what grounds are there to believe that when they come to power in their own little England they will care about (or pay for) a province they clearly regard as a closer, wetter Gibraltar, an irrelevant appendage of the motherland?

Northern Ireland desperately needed a generation of relative political boredom, in which ordinary issues such as taxation and the health service – rather than the unanswerable questions of national identity – could become the stuff of partisan debate. Brexit has made that impossible. Sinn Féin’s immediate call for a referendum on a united Ireland may be reckless and opportunistic, but no more so than the Democratic Unionist party’s failure to understand that Brexit is the best gift to Irish nationalists. It is the beginning of the breakup of the union and the rise of an independent England for which Northern Ireland will be no more than a distant nuisance.

When they take power, the Brexiters have a moral duty to think deeply and speak honestly about these effects of their victory. But the signs are that they will pay as much attention to them as gung-ho warriors typically give to any other kind of collateral damage.