What happened
One of the Vote Leave campaign's clearest promises was that Brexit would not create a hard border between Northern Ireland and the Republic of Ireland. This was presented as straightforward. It was not.
The Irish border problem is structural. Northern Ireland and the Republic share a 310-mile land border with 275 crossing points. Under EU single market rules, goods crossing from a non-EU country (the UK) into an EU member (Ireland) require customs checks. A hard border — physical infrastructure, lorry checks, cameras — would threaten the Good Friday Agreement, which depends on the border being invisible.
The only way to avoid a hard Irish border was to keep Northern Ireland inside the EU's customs and regulatory framework. This is what the Northern Ireland Protocol (and later the Windsor Framework) did — but it created a de facto trade border between Great Britain and Northern Ireland in the Irish Sea instead.
Boris Johnson had personally assured DUP leader Arlene Foster that there would be 'no checks between Northern Ireland and the rest of the UK'. He then signed a protocol that created exactly those checks. He later claimed he hadn't read the protocol. When pushed, he said he'd 'read it differently'.
The practical effect: NI businesses importing from GB now faced EU import paperwork. Supermarkets faced supply disruptions. The DUP collapsed the Stormont power-sharing executive in protest. Northern Ireland's constitutional position became a permanent crisis that the Windsor Framework of 2023 partially addressed but did not resolve.