LONDON — Much of the stated opposition to Prime Minister Theresa May's divorce deal with the European Union centered on the "backstop." The provision was designed to prevent the reintroduction of border controls between Northern Ireland, which is part of the U.K., and the Republic of Ireland, a member of the EU. After Brexit, the border will be the U.K.'s only land frontier with the EU.

A look at the issue:

WHAT'S WRONG WITH A BORDER?

During the decades of sectarian violence in Northern Ireland known as "the Troubles," a border with roadblocks and checkpoints teemed with soldiers and paramilitaries. About 3,700 people were killed in a conflict between Irish nationalists and U.K. unionists from 1968 to 1998, when the Good Friday accord led to a power-sharing arrangement that quelled much of the bloodshed and made the border all but disappear.

Since both Britain and Ireland are currently part of the European Union with its single market, people and goods flow freely between Ireland and Northern Ireland., with no need for customs checks. Brexit could disrupt that easy movement, upending lives and businesses, and undercutting a fragile peace process.

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WHAT WAS MAY'S PROPOSAL?

The proposed withdrawal agreement included a "backstop" provision to keep a hard border from returning by keeping the U.K. in a customs union with the EU after Brexit. The agreement gave Britain and the EU until 2022 to reach a new permanent trade deal and stated the "backstop" would come into effect only if they failed to do so.