Brexit is about drawing lines on maps and hearts. In that respect it is at odds with the 1998 Good Friday agreement which sought to erase them. Key to ending the Troubles was the removal of the hard border between the north and south of Ireland. Theresa May’s problem is that she has committed to leaving the European Union while respecting the peace deal that depended on both London and Dublin being part of it. The agreement made less salient the questions of identity: under its provisions people in Northern Ireland could be, uncontroversially, citizens of Britain or Ireland. What did it matter if they were part of a club that allowed people to travel, work and reside anywhere? North-south institutions were set up to oversee common endeavours. The agreement was sold as a “stepping stone to a united Ireland” to republicans, while unionists held that it was “securing the union”. The symbolism was key: with a hard border you can see that the island is partitioned, without it you cannot.

Aware of this, London and Dublin committed not to return to the days of border checkpoints, which would risk undermining the peace deal’s principles. Mrs May’s withdrawal agreement enshrines this in law in the form of an insurance policy: to maintain an open border on the island of Ireland in the event that the UK leaves the EU without securing an all-encompassing deal, there would be a backstop arrangement to allow for frictionless trade. Fanatical rightwing anti-EU Tory MPs were not bothered about the peace process, or the aspirations of fellow subjects. Instead, they saw in the insurance policy a devious mechanism to force Britain to march in lockstep with EU regulations that hard Brexiters were desperate to get out of. To reverse her government’s historic Commons defeat, Mrs May voted to replace the backstop. She did so to win over the Democratic Unionist party, but her actions enraged businesses in Northern Ireland which had publicly backed her deal.