We are told that this will be the year Brexit is set in motion. That could be good or bad, depending on your perspective. But in any event, we are overlooking a serious point. The problem with “Brexit”, as shorthand for “British exit”, is that there is no such place as Britain.

Once upon a time there was a Roman province called Britannia, but it did not include Ireland, or Scotland north of Hadrian’s Wall. What does exist today is a state called the United Kingdom of Great Britain and Northern Ireland. It is not a nation state like France or Denmark. It is a multinational state, like the former Yugoslavia or the Austro-Hungarian empire.

It is also a distinctly odd state. It contains four nations, one far richer, more populous and more powerful than the other three. England contains almost 84% of the UK population, Scotland has just under 8.5%, Wales just under 5%, and Northern Ireland just under 3%. The state populations of Germany and the United States vary widely, but none towers over the rest to remotely this extent. To judge by what they say and do, the London-centred political and media elites are blind to the tangled history this oddity reflects. As a result the union state is now heading for break-up.

The story of the United Kingdom, the story that should shape the current debate but hasn’t thus far, is one of relentless English expansion – sometimes peaceful and sometimes warlike – at the expense of the non-English nations of the Britannic archipelago. Northern Ireland is the legatee of a war of conquest between the English crown and the native Irish, which ebbed and flowed for centuries and ended with the island’s partition between the overwhelmingly Catholic south and west and the predominantly Protestant north-east.

Anti-Brexit protesters in Cardiff. Photograph: Matthew Horwood/Getty Images

Wales was conquered by the English crown in complex medieval struggles, in which Welsh warlords were as apt to fight one another as to fight the English; the title “Prince of Wales” is a reminder of the English victory, not an emblem of Welsh defiance. Scotland was never conquered, despite brutal incursions by Edward I and Oliver Cromwell. But the Act of Union of 1707 that incorporated Scotland and England (an England that included Wales) into the new polity of Great Britain provoked fierce resistance in Scotland, overcome only by a mixture of threats and bribes.

It is not a pretty story: state-building is rarely a pretty process. Yet for much of the past two centuries, most of the inhabitants of the union state have taken it for granted; it has been in serious contention only in Ireland. The “mystic chords of memory” that Abraham Lincoln evoked in his first inaugural speech, when he hoped to forestall the secession of the southern states, have pulled three of the four nations of the United Kingdom together – thanks, not least, to its astonishing success as an imperial predator.

Now all is changed. The empire on which the sun never set has been consigned to the dustbin of history. Welsh steelworkers’ jobs depend on decisions taken by an Indian conglomerate, headquartered in Mumbai. More important, Lincoln’s mystic chords have been pulling the nations of the UK apart, instead of together, for nearly 20 years.

The first term of the Blair government saw a Scottish parliament sitting in Edinburgh for the first time since 1707, and a Welsh assembly sitting in Cardiff for the first time ever. The Good Friday agreement set up a new form of “power sharing” in Northern Ireland. There are now four capital cities, four administrations and four legislatures in the once monolithic union state. Each of the devolved administrations has its own political priorities and follows its own political trajectory.

Pro-EU campaigners outside the Scottish parliament in Edinburgh. Photograph: Andy Buchanan/AFP/Getty Images

In the last Westminster election, different parties or groups of parties won majorities in each of the four nations, for the first time in history: the Scottish National party won overwhelmingly in Scotland; the Labour party won comfortably in Wales; the Conservatives won easily in England; and the Democratic Unionists were the largest party in Northern Ireland.

Like a mortar bomb crashing into a building infested with dry rot, the EU referendum has torn great holes in the structure, the operational codes and the underlying assumptions of the increasingly rickety union state. Two of the union’s four nations voted to leave the European Union; two voted to remain. There were dramatic differences within three of the four. The Scottish vote to remain was pretty uniform across the nation. But in Northern Ireland, Sinn Féin areas voted to remain while DUP areas voted to leave. England voted to leave but London, Liverpool and Manchester voted to stay. Wales also voted to leave but Cardiff, the Vale of Glamorgan and Monmouth in the south, and Ceredigion and Gwynedd in the north, voted to remain.

Meanwhile the constitutional status of the referendum – and therefore of the leave majority – is in dispute. Since the UK is a parliamentary democracy, referendums can only be advisory. It is up to parliament to decide whether or not to accept the advice. But one thing is clear: the union state is not a unitary state, still less an English state. It would be outrageous to force Scotland and Northern Ireland to leave the EU in defiance of the wishes of their peoples. What is sauce for the English and Welsh leavers is also sauce for the Scottish and Northern Irish remainers.

The Scottish government has every right to hold another independence referendum; and now that the oil price is rising again, the only prudential argument against Scottish independence within the EU has lost its force. Equally, Northern Ireland has every right to rejoin the rest of the island of Ireland – a right guaranteed by the 1998 Good Friday agreement. Theresa May now faces one of the hardest choices ever faced by a British prime minister. She can strain every nerve to make sure that Scotland and Northern Ireland remain in the EU while also remaining in the UK – difficult but not impossible.

If, however, Scotland leaves the EU, along with a Northern Ireland rendered unstable once again by this week’s resignation of Sinn Féin’s Martin McGuinness as deputy first minister – with a consequent threat to power sharing – Wales will be yoked for ever to a Conservative-dominated England. That will stick in the throats of all self-respecting Welsh people. The United Kingdom will be of past tense. To neglect this bigger picture in the months to come would be a monumental mistake.