Britain’s Parliament has told Prime Minister Theresa May she can file for divorce from the European Union. She will send the formal letter by the end of March. Then comes the hard part the arguments, the lawyers, the squabbles over money.

What is the E.U. and why is Britain leaving?

The E.U. is a bloc of 28 nations sharing relatively open borders, a single market in goods and services and for 19 nations a single currency, the euro. Britain joined in 1973 but has long been a somewhat reluctant member, with a large contingent of euroskeptic politicians and journalists regularly railing against regulations imposed by the E.U.’s headquarters in Brussels.

Former Prime Minister David Cameron offered voters a referendum on E.U. membership, and in June 2016 they voted by 52%-48% to leave.

A Union flag, and a European Union flag fly above Europe House, the U.K. Information Office of the European Parliament, in London on Monday. | Photo Credit: AFP

The bill passed by Parliament late March 13, 2017 authorises the British government to invoke Article 50 of the E.U.’s Lisbon Treaty, which says a member state may “notify the European Council of its intention” to leave the bloc.

Later this month, Ms. May is expected to send the notification in a letter to Council President Donald Tusk and then announce the news, probably to Parliament.

That sets a clock ticking — Article 50 says that two years from the moment of notification, “the Treaties shall cease to apply” and Britain will no longer be an E.U. member.

Whose move is it now?

The timing of Article 50 was up to Britain. What happens next is up to the E.U.

Mr. Tusk says that that once E.U. officials get Britain’s notification, they will respond within 48 hours, offering draft negotiating guidelines for the 27 remaining member states to consider. Leaders of the 27 nations will then meet to finalise their negotiating platform; if Article 50 is triggered this week, the meeting will be on April 6, 2017.

“Then we meet and we start,” U.K. Brexit Secretary David Davis said on Sunday. “And I guess the first meeting, bluntly, will be about how we do this? How many meetings, you know, who’s going to meet, who’s going to come.”

Substantial talks may have to wait until after France’s two-round April-May election for a new President. Another hiccup could be Germany’s September election, which will determine whether Chancellor Angela Merkel gets another term.

Who conducts the negotiations?

On the British side, Mr. Davis will take the lead, reporting to Ms. May. Britain’s ambassador to the E.U., Tim Barrow, will also play a major role, and the Foreign Office will talk to individual member states to try to get them on its side.

On the E.U. side, it’s complicated. As Britain’s Institute for Government recently pointed out, “the U.K. is negotiating with 27 member states, not a unified bloc.”

French diplomat Michel Barnier is the chief negotiator for the European Commission, the bloc’s executive arm. He’ll receive direction from the Council, which represents the leaders of the member states.

The European Parliament also wants a say, and will have to approve the final deal between Britain and the bloc.

What is the most pressing issue?

Britain’s vote to leave the E.U. has meant uncertainty for 3 million E.U. citizens living in the U.K., and 1 million Britons who reside in the 27 other nations of the bloc. Both sides agree that providing a guarantee that they will be able to stay where they are is a top priority.

What will be the main conflicts?

The first major battle is likely to be about money. The E.U. says Britain must pay a hefty divorce bill of up to €60 billion ($64 billion), to cover E.U. staff pensions and other expenses the U.K. has committed to. Britain hasn’t ruled out a payment, but is sure to quibble over the size of the tab.

There’s also likely to be friction over Britain’s desire to have free trade in goods and services with the bloc, without accepting the E.U.’s core principle of free movement of workers. Britain has said it will impose limits on immigration, and so will have to leave the E.U.’s single market and customs union. That makes some barriers to trade seem inevitable.

When will it be over?

Under the terms of Article 50, Britain will cease to be an E.U. member in March 2019.

But E.U. negotiators warn it could take two years just to settle the divorce terms; agreeing a new relationship between the U.K. and the E.U. could take years longer. If the rest of the E.U. agrees, the two-year negotiating period can be extended, with Britain still in the E.U. Or, the two sides could agree on a transitional period.

There’s also a chance Britain could walk away early without a deal if it thinks talks are going nowhere.

Is Brexit a one-way ticket?

The British government has said firmly that it will not backtrack on Brexit. But it’s unclear whether Article 50 is legally reversible. Former British ambassador to the E.U. John Kerr, who wrote Article 50, says “it is not irrevocable. You can change your mind while the process is going on.”