Is the Good Friday Agreement under attack? This week David Davis and Karen Bradley have had publicly to reaffirm the Government’s commitment to the deal, whose 20th anniversary is in April. Given the near-canonical status of the text in the modern British state, it is mildly shocking for ministers to have to say out loud that they still support it. Why were they forced to do this?

In part, because of the depressing collapse of power-sharing talks in Belfast and the prospect of direct rule being imposed from London. But also because many committed Brexiteers understand that standing between them and the dream of liberation from the EU lies the Good Friday Agreement. In recent days the MEP Daniel Hannan, the former Northern Ireland secretary Owen Paterson and Labour’s most trenchant Leaver (other than its leader) Kate Hoey have suggested the agreement has outlived its purpose.

The purpose of the accord is commonly misunderstood as being simply about the internal governance of Northern Ireland. It is in fact an international treaty between the UK and Ireland. Until the agreement both the UK and Republic of Ireland claimed jurisdiction over Northern Ireland. This was a product of Irish partition, which gave independence to most — but not all — of Ireland. The majority — but not all — of those living in Northern Ireland wanted to remain within the UK.

To describe Northern Ireland is to describe a puzzle. Under the agreement Northern Ireland will remain in the UK until a majority vote otherwise, but its citizens have the legal and permanent right to be British, Irish or both. The Irish government retains a say in the running of Northern Ireland.

This is why hard Brexiteers have set their sights on the Good Friday Agreement: because it implies the UK’s sovereignty cannot be untrammelled, even after Brexit. UK sovereignty is, in one corner of the realm, qualified by the rights of one group of citizens and by another EU member state that has a say in the administration of that region.

We are often told that Brexit is really a very simple thing: a nation asserting its right to independence. But which nation? This weekend Ireland will play Wales in the Six Nations rugby tournament. The Ireland competing is the island of Ireland, part of which is in the UK —and therefore also part of the nation asserting its sovereignty by leaving the EU. If that strikes you as puzzling, you are beginning to understand why the Good Friday Agreement exists — and why some Brexiteers detest it.

Matthew O’Toole was chief press officer for Europe in 10 Downing Street. He now works for the Powerscourt Group, a communications consultancy