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.
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.
WHY DID CRITICS OPPOSE IT?
Politicians favoring Brexit complained that Britain wouldn't be able to get out of the backstop unilaterally; the deal required the mutual agreement of both sides. That meant it could remain in place indefinitely and keep the U.K. bound to EU customs regulations.
Critics argued such a scenario would derail Britain's efforts to strike other international trade deals. Lawmakers who want to remain close to the EU disliked it, too, because Britain would be subject to customs and trade rules over which it had no say.
May's political allies from Northern Ireland, the Democratic Unionist Party, also objected because the backstop treated Northern Ireland differently from other parts of the U.K. The party said that frayed the bond between Northern Ireland and the rest of the country.