But if Britain leaves the E.U., it could potentially need to reestablish customs controls and entry points with Ireland. Many fear that returning such checkpoints to the border between Northern Ireland and Ireland — making it a “hard border” once more — would be reminiscent of the island’s dark past, potentially unleashing a new wave of instability.

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“Both the U.K. and Ireland will have an obligation to honor the Good Friday Agreement, protect the peace and honor our commitment to the people of Ireland and Northern Ireland, that there won’t be a hard border,” Irish Prime Minister Leo Varadkar said Wednesday.

Former U.S. senator George J. Mitchell, who served as independent chairman of the Northern Ireland peace talks, wrote in The Washington Post in December that changing the status of the border “could increase the possibility of a resumption of violence.”

Those living there have similar concerns. As William Booth and Amanda Ferguson wrote for The Washington Post in November, some people just over the border in Northern Ireland “worry that a bungled Brexit could rekindle tensions and possibly lead to violence.” In recent days, hundreds of people have gathered by the border, holding signs and protesting the possibility of new restrictions.

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But attempts to solve the issue have ground the Brexit process to a halt.

The withdrawal deal that British Prime Minister Theresa May struck with the E.U. included a provision known as the “backstop” to make sure that a hard border would not be reinstated in Ireland. The backstop specified that Northern Ireland would leave the E.U. customs union only when Britain and the E.U. were able to work out an arrangement to avoid a hard border. It was intended to give the two sides time to hash out a final deal on the issue after Britain leaves the block on March 29.

Instead, the backstop enraged May’s coalition partners, Northern Ireland’s Democratic Unionist Party, and many in her own Conservative Party. The dissidents argued that the backstop would essentially dismember Britain, placing Northern Ireland outside of it indefinitely.

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After May’s deal went down to overwhelming defeat in January, the House of Commons voted to send her back to Brussels to try to secure “alternative arrangements” for the Irish border, prompting fears that those arrangements could include a hard border.

“What they have done is they’ve reneged on the backstop and reneged on the Good Friday Agreement,” Ian Blackford, a lawmaker from the Scottish National Party, said Tuesday.

May responded the following day, reiterating “the commitment of this government to the Belfast Good Friday agreement” and adding that Blackford’s remarks were “frankly irresponsible.” Blackford quipped back that “the only thing that is irresponsible is the actions of this prime minister.”

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A hard border remains unlikely. European Commission President Jean-Claude Juncker said Wednesday that renegotiating the Brexit deal is not a possibility, as has the Irish government. Still, even the suggestion of such a move has many in Ireland on edge.