How Ireland's Border Became a Bigger Brexit Issue: QuickTake Q&A

The border between the Republic of Ireland and Northern Ireland will be the only land crossing between the U.K. and the European Union after Britain leaves the bloc. How it’s managed has become one of the key questions to be resolved in the Brexit negotiations, with both sides agreeing that people and goods should be able to move seamlessly back and forth. Now that the U.K.’s Conservative Party is turning to Northern Ireland’s Democratic Unionist Party to form a government after the June 8 election, the border is likely to take on a more central role in the talks. Nearly two decades after the end of a conflict that claimed 3,500 lives, worries remain that a so-called hard border between north and south could lead to a return to violence.

The 310-mile (500 kilometer) border has existed since 1922, when the island was partitioned as part of a peace agreement between the U.K. government and Irish rebels seeking independence. As part of the deal, Northern Ireland, where the population is majority Protestant, remained part of the U.K., which binds together England, Scotland and Wales. The mostly Catholic southern part of the island became the Irish Free State, and gained full independence in 1948.