A trade dispute between the U.K. and the EU erupts over post-Brexit deal

A truck arrives at Larne port in County Antrim, where a customs post has been established as part of the Northern Ireland Protocol, on November 29, 2021. (Paul Faith/AFP via Getty Images)

When the United Kingdom officially left the European Union, Northern Ireland stayed behind in one significant way.

The country effectively remained part of the EU's single market for goods, a concept that allows goods to move freely among the member states. That condition was called the Northern Ireland Protocol.