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How Did the Irish Border Become the Biggest Fight of Brexit?

The Blacklion bridge crosses the Belcoo River which marks the border between Northern Ireland and Ireland.
The Blacklion bridge crosses the Belcoo River which marks the border between Northern Ireland and Ireland.Photographer: Mary Turner/Bloomberg
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In the run-up to the U.K.’s 2016 Brexit referendum, the Irish border barely featured in the debate. Two years on, the question of how to keep the 310-mile (500-kilometer) frontier free of checkpoints and patrols threatens to derail Britain’s plans for an orderly departure from the European Union. How did we get here? And what happens next?

Long controlled by Britain, Ireland was split in 1922 into the Catholic-dominated south and the mainly Protestant north. The south won independence, the north remained under British rule. For decades, customs officers patrolled the new border to stop smugglers. Later, soldiers manned heavily fortified checkpoints and watchtowers on the border, as violence raged across the region.