After Brexit, the border will be the U.K.'s only land frontier with the EU, but this is a frontier with huge political as well as economic significance. 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 the conflict between Irish nationalists and U.K. unionists and the British government, from 1968 to 1998, when the Good Friday accord led to a power-sharing arrangement. That quelled much of the bloodshed and brought an end to the military checkpoints on the border.
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 checks. Brexit could disrupt that easy movement, not only upending lives and businesses but also potentially undermining the fragile peace process.
WHAT IS THE 'BACKSTOP' PROPOSAL?
To prevent the reinstatement of a hard border on the island of Ireland, with customs checks at the frontier, the proposed withdrawal agreement included a "backstop" provision that would keep 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, with the backstop coming 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. They 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.
Prime Minister Boris Johnson's political allies in Northern Ireland, the Democratic Unionist Party, object because the backstop would treat Northern Ireland differently from other parts of the U.K. The party said that frays the bond between Northern Ireland and the rest of the country.
HOW COULD THE BACKSTOP ISSUE BE SOLVED?
Johnson has demanded that the backstop be removed from the deal, but EU leaders refuse. He is expected to submit detailed alternative plans in the coming days.
So far the U.K. has floated the idea of a common area for livestock and agricultural products, plus largely untested "technological solutions." The EU says that is inadequate.
Ireland's deputy prime minister has rejected an idea raised in preliminary U.K. proposals for customs posts 5 to 10 miles (8 to 16 kilometers) away from the border. Johnson has said this will not be included in his proposals.