Three nations, one union under strain: The long history behind United Kingdom's latest breakup threat

The United Kingdom did not emerge as a single state in one moment. It was assembled through a series of unions, legislation, conquest and political settlements spread over several centuries.

Now, with the leaders of the Scottish National Party, Plaid Cymru and Sinn Féin signing a memorandum of understanding in Cardiff calling on Westminster to “prepare, plan and facilitate constitutional change”, the question of whether the UK can remain intact has moved back into the spotlight.

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The pact has political significance, but it does not itself change the UK's constitutional arrangements. The leaders signed it as party leaders rather than representatives formally binding their governments, and an expert quoted by AP said the agreement had no legal force.

How did the United Kingdom come together?

Wales came first. Wales had been conquered by the English Crown over the 13th century. The Laws in Wales Acts of 1535 and 1542 formally incorporated Wales into the English legal and political system, giving it representation in the Parliament of England.

Scotland followed, but through a different route. In 1603, the death of Elizabeth I brought James VI of Scotland to the English throne as James I. England and Scotland therefore shared a monarch, but remained separate kingdoms, with separate parliaments and legal systems. This was the Union of the Crowns, not yet a political union.

That changed in 1707. The English and Scottish parliaments approved the Acts of Union, creating the Kingdom of Great Britain from May 1, 1707, with a single Parliament. Scotland retained important separate institutions, including its distinct legal system and church.

Ireland joined later. Following the 1798 rebellion and concerns about French intervention, the British and Irish parliaments passed the Acts of Union. They came into force on January 1, 1801, creating the United Kingdom of Great Britain and Ireland and abolishing the Irish Parliament. Ireland received 100 MPs at Westminster.

But the Irish union did not last in its original form. The Government of Ireland Act 1920 partitioned the island, creating separate arrangements for Northern and Southern Ireland. The Anglo-Irish settlement that followed the Irish War of Independence led to the creation of the Irish Free State, while Northern Ireland remained within the UK. The modern United Kingdom therefore became the United Kingdom of Great Britain and Northern Ireland.

The next major transformation came in 1998, when the Good Friday Agreement restored devolved government to Northern Ireland. The same period produced the modern Scottish Parliament and Welsh legislature through the Scotland Act and devolution legislation.

What does the new pact actually say?

The SNP, Plaid Cymru and Sinn Féin argue that Scotland, Wales and Northern Ireland have the right to determine their own constitutional futures. The agreement calls on Westminster to prepare for and facilitate constitutional change and says the political landscape across the UK and Ireland is changing.

It is significant because, for the first time, pro-independence or pro-unification parties lead all three devolved administrations. But their objectives are not identical.

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The SNP wants another Scottish independence referendum. Sinn Féin wants a referendum on Irish unity when the constitutional conditions for one are met. Plaid Cymru supports Welsh independence but has not set the same immediate referendum timetable.

The separate Cardiff Agreement between the Scottish and Welsh governments covers practical cooperation on issues including poverty, living standards, economic growth, climate policy and international engagement with the EU. It does not itself begin a process of independence.

Can the UK actually break up?

Yes, but not because of this pact. There is no single constitutional document that sets out one simple procedure for dissolving the UK. The House of Commons Library describes the country as a “state of unions” that has developed gradually over centuries.

For Scotland, the biggest immediate obstacle is Westminster. In 2022, the UK Supreme Court ruled that the Scottish Parliament did not have the power to hold an independence referendum on its own because the question directly concerns reserved matters, including the Union and the UK Parliament.

That means a legally authorised referendum would require agreement from Westminster, as happened before the 2014 vote. Scotland voted 55% to 45% to remain in the UK then.

Northern Ireland is different. Under the Good Friday Agreement, the British government is required to hold a border poll if it appears likely that a majority would vote for Northern Ireland to leave the UK and join a united Ireland. A corresponding vote would also be required in the Republic of Ireland.

Wales has no equivalent automatic mechanism for independence. Any move towards statehood would require a political and legal process involving Westminster.

So even if nationalist parties win elections, an election victory does not itself dissolve the UK. Referendums, legislation and negotiations over assets, debt, borders, pensions, citizenship, trade, defence and international treaties would follow.

Why Scotland matters so much for energy

The breakup question is not simply about flags and constitutional law. Scotland occupies an important place in the UK's energy system.

Scotland generated a record 38.5 TWh of renewable electricity in 2025, with wind accounting for 30.8 TWh. It had 17.9 GW of operational renewable capacity and recorded net electricity exports of 20.8 TWh that year.

That means independence would create complicated questions over electricity markets, transmission networks, offshore projects, North Sea resources and regulation. It would not automatically leave England and Wales facing an energy crisis: the existing electricity system is interconnected, and a future Scottish state and the remainder of the UK would have strong incentives to maintain cross-border trade.

But the ownership and revenues from North Sea oil and gas, as well as rapidly expanding offshore wind, would become major issues in negotiations.

Then there is Britain’s nuclear deterrent

The most difficult defence question could be Trident. Britain's nuclear-armed submarines operate from HM Naval Base Clyde at Faslane, while nuclear warheads are handled and stored at nearby Coulport. The UK government says Faslane is home to its nuclear deterrent patrol submarines and that Coulport handles the storage and loading of Trident warheads.

An independent Scotland would have to decide whether those facilities could continue hosting Britain's nuclear force. The SNP has historically opposed nuclear weapons and has called for their removal from Scotland.

Relocating the deterrent would be extremely complicated. A House of Commons inquiry concluded that recreating facilities equivalent to Faslane and Coulport elsewhere would be highly problematic, expensive and politically difficult. The Ministry of Defence at the time described the project as one that would take a very long time and cost huge sums of money.

The precise cost is disputed, so claims that relocation would necessarily cost more than £20 billion should be treated cautiously.

What happens next?

The Cardiff pact does not mean the United Kingdom is about to disappear. Its immediate importance is political. Scotland, Wales and Northern Ireland's nationalist movements are increasingly coordinating their demands, while Brexit has strengthened the argument in parts of those countries that their political interests diverge from Westminster's.

The next test is whether that political pressure translates into sustained majority support for constitutional change, and whether Westminster eventually agrees to give voters the legal mechanism to act on it.

For now, the UK remains intact. But the question that has followed it since the unions of 1707 and 1801 is once again back at the centre of British politics. The questions on how much consent is required to keep a union together and what happens when that consent begins to weaken? is reappearing.