t was only Achilles’ heel that was vulnerable, but it still killed him. The Northern Ireland border was Boris Johnson’s greatest Brexit “untruth”. He told the taoiseach there would be no border in Ireland. He told Northern Ireland’s unionists there would be none in the Irish Sea. He told everyone he would leave Europe’s customs union. Johnson was, as the saying goes, “averse to the despotism of facts”. The latest car crash is Johnson’s decision to abrogate yet again the Northern Ireland protocol with the EU. He is refusing to regulate a customs border in Belfast. Since this implies breaching EU customs control, Brussels is angry and is threatening legal action “very soon”. We have previously seen problems with some food supplies getting to Northern Ireland as a result of the end of the initial Brexit transition period at the start of the year. Trade is not about chauvinist rhetoric but about people’s lives. As the “grace periods” come to an end, it is clear that Johnson must quickly decide what to do, whether to erect customs barriers around the six counties, incidentally breaking the Good Friday agreement, or erect them around Belfast dock. The first would be a logistical and emotional nightmare. It would probably drive the north’s eventual reunion with the south – and much strife in the process. Or he must do the EU’s bidding, barricade Belfast and enrage the unionists, which he has twice refused to do in illegal breach of the Northern Ireland protocol. He simply must decide. This weekend, Johnson’s favourite undiplomatic diplomat, David Frost, told the EU to stop moaning and “shake off remaining ill will” towards Britain. The ill will was entirely of Johnson and Frost’s making, a relentless story of mendacity and belligerence throughout the Brexit talks. The EU was never going to make it easy for the UK to leave. It had half a dozen other countries within the bloc to discourage and laboriously negotiated trade deals with many other external states to protect. Besides, the real issue was not Brexit itself. The issue was leaving the customs union – which is the precise trouble in Northern Ireland. Versions of such a problem have existed ever since partition in 1922. It was why Johnson should have levelled with the people of Northern Ireland from the start. Frost’s haphazard negotiations left unresolved everything from Scottish fishing to City banking, policing and cultural exchanges. The Johnson/Frost line that “the EU needs us more than we need it” was always rubbish. But nothing was so glaring as the contradiction of trade with Northern Ireland. Britain could leave the customs union. Northern Ireland realistically could not. Johnson has been boiling fudge ever since. Now the pan is burning. The simple and honest route is to ask Northern Irish voters what they want. They were never asked if they wanted to leave the customs union – any more than was the rest of the UK. That was Johnson’s idea. One day Britain will almost certainly reverse the idiocy and rejoin. Until that happens international law is clear, a border there must be. So ask Northern Ireland where it should be. Simon Jenkins is a Guardian columnist