Brexit deal has made the question of the Irish border a live political issue.

I met pleased and gloomy people in the first half of last year when I travelled around the UK writing about the potential impact of Brexit. But by far the happiest of those I interviewed were veteran Irish republicans in Belfast, mostly present or past members of Sinn Fein, who had devoted their lives to opposing British rule.

They grasped that Brexit had made the question of the Irish border a live political issue by turning it into an international frontier. This was no longer just a 310-mile-long dividing line between the UK and the Irish Republic but the border between the UK and the EU.

Irish nationalists had been trying to interest the rest of the world in the partition of Ireland since it happened in 1921 but had failed dismally. Now the British government was self-destructively doing their work for them, significantly eroding the status of Northern Ireland as part of the UK.

Unionist fears that they would be sold out have been amply fulfilled, an outcome confirmed this week by the EU-UK government deal on the Irish Sea border, which is complicated and confusing – perhaps deliberately so – but means in practice that there will be trade barriers between Northern Ireland and Britain, though not between the Northern Ireland and the Irish Republic. Unionist papers now refer to an “Irish Sea border” as a done deal. At the same time, Boris Johnson dropped the much-criticised clauses in the Internal Market Bill that would have enabled his government to renege on the sea border.

Former leaders of the Brexiteers, propelled into power by claiming to be “taking back control”, are coy about what they have done, but the British state is ceding a large measure of authority over part of the UK. Northern Ireland will in future abide by EU customs regulations and “single market” rules and the rest of the UK will not. Unsurprisingly, the pro-Brexit British media that has been booming defiance towards Brussels in the last few days and telling Johnson to stand firm is largely mute about this diminution in the real power of the British government.

The unionists in Northern Ireland are not so shy in expressing their sense of betrayal. “The prime minister will always do what is right for him personally, then for his party, then for England,” says a bitter editorial in the unionist News Letter quoted by The Irish Times. “He showed pure cynicism when he came to Northern Ireland to denounce Theresa May’s border backstop … Within months he had the premiership he had spent a lifetime coveting, and within weeks of achieving that goal he was cutting the province adrift.”

Of course, anybody putting their trust in anybody so openly duplicitous as Johnson does not deserve much sympathy. Arlene Foster and the Democratic Unionist Party, for all their reputation for ruthless pragmatism, showed child-like naivety in imagining that the Conservative Party’s embrace would last a second longer than its dependence on the DUP for its majority in the House of Commons.

But the tortured emergence of the “Irish Sea border”, together with the dropping of the disputed clauses in the Internal Market Bill, does more than provide further example of Johnson’s untrustworthiness. They are the outcome of changes in the balance of power on a number of fronts, all negative for Britain. The first is between Protestants/unionists and Catholics/nationalists in Northern Ireland. The dominance of the former has been slipping away since the start of the Troubles in 1968, a decline recognised and institutionalised by the Good Friday Agreement 30 years later.

The balance between the two communities will change further as the unionists/Protestants lose their demographic majority in the upcoming census.

The outcome of the struggle over the location of the Irish border is more than just an Irish issue; it has been a crucial test of strength between post-Brexit Britain and the Irish Republic, the EU itself, and the US as Joe Biden becomes president. The result of the test is to show that global Britain is a shrunken power, regardless of the outcome of the current trade talks.

Nationalists suffer from two weaknesses in particular: they blame everything that goes wrong on an external oppressor, such as, in Britain’s case, Brussels. Second, they are blind to the nationalism of others, seeing it as phoney and inspired by base motives like economic greed, and entirely different from their own pristine patriotism. The Brexiteers never considered how their own English nationalism would affect Ireland and Scotland. Yet the last 15 opinion polls in Scotland show that a majority of Scots now favour independence. Even if this does not happen soon, the “Scottish question” is going to be a dominant theme in British politics in the coming decades, just as the “Irish question” once was.

We are not yet seeing the final break-up of the UK, but the ebbing of English power within the British Isles as a whole is becoming ever more visible. Ireland has a thirteenth of the population of the UK but has become a political rival to be taken seriously because it is backed, at least for the moment, by the EU and the US. No wonder those Sinn Fein leaders in Belfast were looking so happy.