A new fight has been brewing over the consequences of Brexit for the border between Northern Ireland and the neighboring Republic of Ireland. Both British and EU negotiators have identified the border as one of the key political questions that have to be addressed early on in negotiations. It’s easy not to have a border between two states that are both part of the EU’s single market (the regulatory regime that allows easy trade across the EU) and customs union (the arrangement under which European states do not impose customs duties on each other and maintain a common