It was a Frenchman who recently observed that every nation has its own massive, historically intractable problem. For France, he said, this is the stubborn popular opposition to raising the national pension age in response to ever-increasing longevity. For Germany, it is still the spectre of Hitler. For the United States, it is guns. For Britain, it is Ireland.

Last week, fierce controversy broke out within the ruling Conservative Party about “John Bull’s other island”, threatening to rend the prime minister Rishi Sunak’s government asunder. For more than five centuries, British rule over Ireland, almost unfailingly incompetent as well as cruel, inflicted misery on the Irish people and created military and political crises for England. That the English were a Protestant society, while most Irish people were Catholic, made matters worse: until the mid-19th century, England was in a state of almost permanent strife with Catholic Europe.

Then came Irish independence in 1921. It seemed that the boil had been lanced, the issue sort of resolved, despite lingering bitterness on both sides of the Irish Sea. But an historic blunder was committed at the time, when Conservative politicians forced the prime minister David Lloyd George to exclude from the independence settlement the Protestant rump that dominated a portion of Ulster.

A statelet, with a hapless Catholic minority, was carved from the six northernmost counties of Ireland, which remained attached to Britain, while the other 26 counties eventually became the Irish Republic. Today, Northern Ireland has a population of less than 1.9 million, 42 per cent of them Catholic, while five million inhabit the rest of the island, ruled from Dublin.

The Troubles, which for 30 years racked the North with bloodshed, seemed to end with Tony Blair's 1999 Good Friday Agreement. British and Irish membership of the European Union appeared to make much more real the prospect — which many of us English people welcomed — that within our lifetimes Ireland may once more become united.

But Britain’s departure from the European Union has proved a disaster for prospects of Irish togetherness. Former prime minister Boris Johnson negotiated a “Northern Ireland Protocol” as part of his Brexit deal, which granted a special trading status, with provision for customs checks on goods that entered or left Britain via Ulster, while retaining free movement across the Irish border.

Johnson misled the Protestant political class, many of them members of the Democratic Unionist Party, who are obsessed with preserving their link not to the real Britain, but to a fantasy Britain of long ago. He claimed that the deal left their constitutional position unchanged; that there would be no “border down the Irish Sea … over my dead body”. In truth, there had to be customs checks, part of the inescapable cost of Brexit.

The Unionists, enraged by perceived betrayal, last year withdrew from the power-sharing body that has governed Northern Ireland since the 1999 peace settlement. They have boycotted the assembly ever since, partly in disgust that last year’s election made the Republican Sinn Féin party larger than their own grouping for the first time.

Sunak is now obliged to wrestle with the problem of how to square a circle: to fulfil Britain’s legally binding undertaking to the EU, and to sustain the Good Friday Agreement, while averting a Protestant revolt. In theory, the latter prospect should hold no terrors, because the Democratic Unionists are in historic retreat. But the right wing of Sunak’s Conservative Party, as implacable and even fanatical as many US Republicans, remain committed to the Democratic Unionists, even at the potential cost of a trade war with the EU and renewal of the Troubles.

Sunak has spent recent days striving to seal a new compromise deal with Brussels, while preventing a right-wing Tory revolt that breaks up his own party in Parliament. Many of us in England regard all this as madness at a time when our country faces huge problems: a tottering economy; collapsing health service; strikes in schools, hospitals and the rail system. Viewed amid a rational catalogue of British priorities, Ulster simply does not matter much.

But the Conservative Right — ironically including Boris Johnson, who seeks to destroy the deal he signed — are no better friends to the imperatives of reason than is the GOP. Johnson urged the Government last week to persist with the legislation he initiated — in defiance of international law — unilaterally to cancel the Northern Ireland Protocol to the Brexit treaty. He threatens to wield a wrecking ball — doubtless as part of his campaign to regain the premiership.

It remains deeply uncertain whether Sunak, close to striking a pragmatic agreement with Brussels for Ulster’s future relationship with Britain and the EU, will be able to secure the support of his own party to drive any deal through the House of Commons, or instead will be obliged to rely on the votes of the Labour opposition.

If the latter outcome prevails, or worse still if the new deal is sabotaged, the blow to the Prime Minister's authority, as well as to the peace and prosperity of Ireland, will be serious indeed. US president Joe Biden has emphasised his commitment to the Northern Ireland Protocol. If Conservative rebels now wreck the proposed compromise, it is hard to suppose that the White House will be eager to do Britain any favours — and we need a few, heaven knows.

The political divisions in Britain over Ireland are, of course, far from unique. All over the world, we behold traditional loyalties fragmenting, old ideas of patriotism shaken by political divergence. Visiting the southern US last year, I was struck by how often I heard fans of Florida governor Ron DeSantis make the joke, more than half-serious, about their pride to be living in “the independent republic of Florida”.

Brexit has imposed enormous strains on British ideas of nationality, among all except our impassioned isolationists. Last month I met a distinguished English historian who told me with delight that he has just been granted a German passport, to which he is entitled because his mother, a Jew persecuted by the Nazis, came from Berlin. He needs the document, he said, because his work requires him to spend much time in Europe; he wants to escape the restrictions that increasingly weigh upon visiting non-EU citizens.

So does my own daughter. By chance, she was born when we lived in Kilkenny, Ireland, and thus was equally delighted to be able to obtain an Irish passport; she is also seeking them for my grandchildren. Not for a moment would I reproach her, or my historian friend. Our nation has adopted a course that seems to reflect a desire to opt out of the international community that “little Britain” needs. Some folks are prudently seeking avenues through which to opt back in.

If Ireland is today’s big focus of Britain’s continuing constitutional crisis, it is not the only one. This month, Scotland’s first minister, Nicola Sturgeon, announced her intention to quit. The decision was greeted rapturously by English Unionists, who perceive her impending resignation as an historic defeat for the tartan independence movement.

In the short term, they are probably right. Sturgeon was a charismatic politician who thrilled many Scots with her defiance, even disdain, of English prime ministers. Yet her party’s governance of Scotland for more than a decade has been a disaster, incompetent at managing education, health, transport and much else. Polls show Scottish support for independence drastically diminished.

But Britain is still far from escaping a northern breakaway because the demographics lean emphatically towards it. The last referendum on independence, nearly a decade ago, was won by Scottish remainers who were overwhelmingly older. Today, Scots between 16 and 24 are six times more likely to favour independence than are their parents or grandparents, heedless of Scottish dependence on English subsidy for state spending. They simply perceive their culture and values as incompatible with those of the English.

Two years ago in a column, I discussed the possibility of Britain’s fragmentation, and concluded that Wales would not go, Northern Ireland should go, and Scotland may go. Today, I would shift that view somewhat. Wales will remain attached because it would not be viable on its own.

Irish unification remains the island's natural destiny. But for all Dublin’s protestations of enthusiasm, the Irish Government flinches before the prospect of assuming responsibility for a Northern Irish economy that is dependent on costly state aid. It poses some of the same problems, on a much smaller scale, that East Germany inflicted on West Germany at its 1990 reunification.

Marriage to the disruptive Northern Unionists also poses a threat to the Irish Republic’s social and political fabric that daunts many Irish people. But in Ireland as in Scotland, demographics and thus time are on the side of change. As old instinctive Unionists die off, the young favour a different agenda.

Brexit has changed many things. Where once England offered the poorer Scots and Northern Irish a link with a richer and more successful partner, today King Charles III’s nation is languishing, and likely to continue to do so. Within a generation, I believe, Scotland will vote for independence and Northern Ireland for reunification with the South.

England will be a smaller place in consequence, in international influence as well as in landmass. But that is the consequence of choices that English people have made, in a world where localism is in fashion almost everywhere that votes are counted.

• Max Hastings is a Bloomberg Opinion columnist. A former editor-in-chief of the Daily Telegraph and the London Evening Standard, he is author, most recently, of The Abyss: Nuclear Crisis Cuba 1962