Last week saw the collision of two big subplots in the story of the UK’s exit from the EU: the Irish border and Democrats making public statements on a US trade deal. The connection between the two may not be obvious to everyone, so it is worth pausing to take stock. The Irish issue was scarcely mentioned during the 2016 campaign, reflecting blithe confidence that it could surely be sorted out easily. It roared to centre-stage, however, during subsequent negotiations.