The Good Friday Agreement, which brought a measure of peace to Northern Ireland, has to be preserved at all costs. But it is very hard to reconcile with Brexit, and the approach adopted by both the British government and the European Union is making that even harder. It was always the case that Britain’s withdrawal from the European Union would require some sort of control over goods passing between the two. That means having borders, staffed by customs officers. And if the Good Friday Agreement required that the border was not to be between Northern Ireland and the Republic, it logically had to lie between Northern Ireland and the British mainland.