In one part of the UK, people are no longer fretting about the negative consequences which might flow from Brexit once the process creaks into action. That is because the negative effects are unfolding already. That region is Northern Ireland.

Formally speaking, a scandalously-botched green energy scheme was the catalyst for the collapse of Northern Ireland’s power-sharing executive, whose very existence over most of the last decade was a kind of gravity-defying miracle. But every observer agrees that there were deeper reasons for the breakdown of joint administration by Sinn Fein, the standard-bearer of militant Irish republicanism, and the Democratic Unionist Party, for which pro-British Protestants vote.

Relations between Arlene Foster, the First Minister and DUP leader, and Sinn Fein’s Martin McGuinness, her deputy, had become ever more scratchy over many months. And nothing poisoned this odd couple’s relationship so much as the fact they, and the voters under their influence, took opposing stances in the Brexit campaign. This was not a disagreement over finely balanced technicalities; it was a clash over what sort of place Northern Ireland is and where it is going.

Broadly speaking, most Catholic and Irish-nationalist voters abhor Brexit for the good reason that it will harden the inter-Irish border and weaken their connections with the rest of the island. By contrast, many Protestant Unionist voters seem to have seen in Brexit an opportunity to turn the clock back, reinforce the bond with Mother Britain and reduce southern Irish influence over their home region. In a depressing return to the past, the province’s rival communities found themselves in a zero-sum game, with each side seeing gain in what the other wants least.

That’s one reason why the campaign for the Northern Ireland’s March 2 elections is so depressingly rancorous, and why it will be so hard to stitch together a fresh power-sharing arrangement between the victors. None of that means that the region is doomed to return to violence which claimed 3,000 lives between 1969 and 1994, but there is a palpable sense of moving backward, not forward.

Hard Brexit = hard border

The atmosphere was further darkened by Theresa May’s “Hard Brexit” speech of January 17 which may prove to be as an ominous a landmark in the Northern Ireland’s history as the downfall of power-sharing a week earlier.

Once the prime minister had confirmed Britain’s intention of leaving the single market and the customs union, it became impossible to pretend that there would be no toughening of the land border, across which a third of Northern Ireland’s exports move.

The most that the London government can now promise is (to quote the Brexit manifesto, published on February 2) that movement of goods across the border will be kept as “seamless and frictionless as possible” [our italics].

But what is possible may be a long way from what is desirable, according to somebody who knows this subject. Michael Lux, a former head of the European Commission’s customs procedures, put it plainly to a British parliamentary committee: “If Northern Ireland is not part of the EU customs territory, then there is a customs border”. A seamless, control-free border was impossible “at least for Ireland because it is obliged to apply EU law.”

Even moving dogs and horses across the border might be subject to restrictions, Lux warned, hinting at a nightmare for farmers whose property straddles the jurisdictions.

His was a sober, technocratic assessment of what may happen.

In the rough-and-tumble of politics, language is more colourful. Gerry Adams, the Sinn Finn president, reacted to the Hard Brexit speech with a tirade whose bitterness recalled darker times. He said removing Northern Ireland from the EU was a “hostile act” which would destroy the 1998 agreement on which power-sharing in Belfast is based. His line of reasoning wasn’t entirely honest: he conflated leaving the Union with quitting the European Convention on Human Rights. But he was articulating something many Catholic voters feel; their Protestant neighbours seized a chance to help impose a change which they want and most Catholics don’t.

Colum Eastwood, the 33-year-old leader of Northern Ireland’s moderate nationalists, is almost as angry. Warning of “economic devastation” as a likely result of quitting the European system of trade, he said imposing Brexit on Northern Ireland, where 56% of people had voted to stay in the EU, was now exposed as an act of “democratic vandalism”.

Sinn Fein and the Eastwood’s Social Democratic and Labour Party draw the same conclusion. Northern Ireland should seek a special status, allowing it to remain in the customs union. Whatever its other merits, that arrangement would cement the region’s ties with the Irish republic and weaken links with Britain. In other words, it is the exact opposite of what the Democratic Unionists, who have been snuggling closer to the British Conservatives, want for their province.

To see why Northern Ireland’s widening chasm is not good news, it’s worth going back to the fundamentals of its peculiar settlement. Peace in the province has been underpinned by a paradox. Even in a deeply divided land, two or more political camps can disagree deeply over the past and ultimate future and still collaborate pragmatically in the short term. In many situations, that sort of collaboration offers the only alternative to war.

Only a few months ago, pragmatic collaboration was still alive, albeit not very well. Foster and McGuinness were even preparing to go China together to drum up investment. Now the province’s rival communities, and their leaders, are pulling in opposite directions. And it’s all because of Brexit.

Edited by Hugo Dixon