The Derry bombing this weekend is a stark reminder of what is at stake in Northern Ireland with Brexit: decades of peace where arguments are settled with words not weapons. In reopening the question of whether the physical border between the North and South of Ireland might return, Brexit threatens to turn back the clock to a far more dangerous era. That past, where symbols summon deadly emotions, has been revived by English nationalists in the Tory party who appear comfortably ignorant of the Troubles. Erasing the border in Ireland, once dotted with watchtowers and checkpoints, was necessary. But Brexit put the deadly issues of the Irish border and sovereignty back into mainstream debate. Dissident republicans have been blamed by the police for the van bomb attack on a Derry courthouse. Their ideological patrons have long recognised Brexit’s potential to reignite the conflict, with one quoted in academic Marisa McGlinchey’s new study Unfinished Business as saying it was “the best chance we’ve had since 1916”.

That is why Theresa May and the European Union committed to avoiding the return of a “hard border” – physical checks or infrastructure – after Brexit. The UK and Ireland are currently part of the EU single market and customs union, so products do not need to be inspected. 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, Mrs May and the EU said it was necessary to have a backstop arrangement that will allow for frictionless trade. The trouble is that the Democratic Unionist party is divided over how to deal with this insurance policy. The party’s “blood red line” of no border down the Irish Sea means it could accept only the hardest or softest of Brexits. Hence Mrs May continues to talk to the DUP in the hope that the party will make up its mind.

The prime minister ought to have realised months ago that the hard Brexit faction within the DUP has its power base in Westminster. It first asserted itself after power-sharing in Northern Ireland collapsed. It became dominant after Mrs May sought DUP support for her parliamentary majority in Westminster in the wake of the 2017 election. The suspended Stormont assembly provided a forum for sane debate: in August 2016 the DUP’s Arlene Foster signed a joint letter with Sinn Féin’s Martin McGuinness on safeguarding the gains of the peace process and the need to protect “the border [issue], the all-island energy market, EU peace funding and the need to maintain tariff- and barrier-free trade with the EU”.

To win support for her deal, Mrs May must seek to revive such sentiments not just within Ms Foster’s party but also within the wider nationalist community. Efforts in this direction won’t be helped by former cabinet ministers ludicrously suggesting that Brexit would have been resolved if not for splits in the Irish government, or that the UK government should simply wait for the EU to dump Dublin in favour of London. The withdrawal agreement already has a provision for Britain to choose whether to enter the backstop or extend the transition. What Mrs May also needs to do urgently is win back the nationalist community and the growing band of Northern Irish non-tribal voters. Departing the EU could see Brexit become synonymous with defending the province’s place within the UK. The risk to unionism is voters could then reassess their constitutional preference. Few want a hard border; they should not block ways to avoid one. Electoral contingency and political expediency cannot be allowed to break up the UK.