LONDON — The militarized checkpoints that once stood along the border between Ireland and Northern Ireland became flash points for sectarian violence during the Troubles, and no one wants to see their return after Britain quits the European Union in 2019.
But a document released by the British government Wednesday on how to preserve the open border there has underscored the complexity of Britain’s planned departure from the bloc, known as Brexit.
Northern Ireland is a part of the United Kingdom, while Ireland has been an independent nation since 1921. Both are members of the European Union — a shared status that has helped London and Dublin put aside historical differences and develop such a close relationship that border controls have disappeared. Travelers generally do not even know when they have passed from one country to the other.
After Brexit, however, the roughly 300-mile frontier with Ireland will be the United Kingdom’s only land border with a bloc whose economic arrangements, including its single market, it plans to leave. That creates a host of problems.
The customs union allows members to trade freely among themselves while charging a single tariff on some goods from nonmembers. When Britain leaves the bloc, goods crossing the border into Ireland could be subject to varying tariffs. Policing those tariffs could be burdensome.
The same principle holds for the single market. It is maintained through a detailed set of standards that Britain would either have to abide by or face the logistical nightmare of checking goods entering the European Union from its territory.
Adopting the same tariffs and standards as the European Union would clear up a lot of problems but would undermine the supposed purpose of Brexit in the first place, which is to re-establish control over immigration and national sovereignty.
The document published Wednesday represents the first, if somewhat vague, attempt to deal with these problems as they affect the Irish frontier. It rules out the reintroduction of physical infrastructure such as customs posts.
Immigration would not be policed at the Irish frontier, nor would there be passport checks on people entering mainland Britain from Northern Ireland.
The document hints that European citizens would probably be allowed to enter Britain freely and directly from Europe even after Brexit, although they might face some restrictions on their right to work.
The bigger problem is trade.
Wednesday’s document calls on the European Union to agree to a series of waivers for small businesses and farmers, to avoid the need for them to complete customs formalities.
But that is only part of the problem. Even if that were agreed, larger companies would surely face higher costs. The British government is hazy on this point, talking about setting up simplified customs procedures and applying unspecified technologies to track goods.
