In October 2018, then DUP party leader, Arlene Foster, used an interview with the BBC to stress that her party had only one red line in the Brexit process, that “there cannot ever be a border down the Irish Sea, a differential between Northern Ireland and the rest of the UK”. This red line was, she asserted, “blood red” (see Morris 2018). Having campaigned for Brexit in 2016, by 2018 the DUP had been forced into a position of having to insist that Northern Ireland would leave the EU on the same terms as the rest of the UK. On the face of it, it did so from a position of strength unparalleled in its history, furnished by the confidence-and-supply arrangement, agreed with Theresa May's Conservative Party to give May the majority she needed in order to keep a hold on the reins of government. Nevertheless, the DUP ultimately failed to secure its preferred Brexit outcome, and cast Northern Ireland's constitutional future in an uncertain light.

The focus here is on the political tensions that Brexit's critical moment has produced for Ulster unionism, how unionists have sought to negotiate these, and with what potential consequences for Northern Ireland's constitutional future. For our purposes, “unionism” comprises those in Northern Ireland whose primary political identity is British and, more particularly, for whom the maintenance of Northern Ireland's present status as part of the United Kingdom is a (or, indeed, the) primary motivation in their political decision-making. Members of those political parties that seek to protect and promote Northern Ireland's position in the Union designate in the Northern Ireland Assembly as “Unionist”. The largest of these, and the largest party in the Assembly (at the time of writing, by one seat) is the DUP (see Tonge et al. 2014). The decisions, actions and strategic (mis)calculations of this party have been of immediate consequence for the course of the Brexit process, and are therefore of primary interest to us.