Why Ireland's Border Is Brexit's Intractable Puzzle

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The boundary between the Republic of Ireland and Northern Ireland was long the scene of tense checkpoints and violent protest. Nearly two decades after the end of a conflict that claimed 3,500 lives, the undulating border is once again caught up in a bitter division. When British and European Union leaders carry out the split -- Brexit -- that British voters ordered up, the border between Ireland’s north and south will be the only land crossing between the two jurisdictions. For now the border is effectively open, meaning people and goods are free to cross back and forth. Whether it remains that way is the most vexing issue in the divorce talks.