How did it become a mess?

LONDON — Tuesday was supposed to be the day that Britain’s battle over Brexit was resolved. Instead, Prime Minister Theresa May shuttled through European capitals, seeking changes to the deal to try to win over skeptical British legislators before Britain leaves the EU on March 29.

HOW DID WE GET HERE?

Britain joined the European Economic Community — now the EU — in 1973, but has long been an ambivalent member. The U.K. never adopted the euro as its currency, and British politicians have been cool to the bloc’s calls for ever-closer political union.

WHAT’S AT STAKE?

Every divorce involves paperwork. Britain can leave without a deal, but it won’t be pretty. Departure will tear up thousands of laws and rules stitched together over more than four decades, covering every aspect of British life and the economy. If Britain and the EU can’t agree on what will replace them, there could be chaos.

WHAT’S THE STICKING POINT?

With compromises on both sides, Britain and the EU managed to reach agreement on many contentious issues. But one has proved intractable: the border between Northern Ireland and Ireland, which will be the U.K.’s only land border with the EU after Brexit.

WHAT HAPPENS NEXT?

The British government says it plans to bring the deal, with whatever changes May achieves, back to Parliament for a vote before Jan 21. If it passes, it still must be approved by the European Parliament, but that isn’t expected to be a problem.