THE COUNTDOWN HAS BEGUN TO A BREXIT DIVORCE. AND IT COMES AT A TIME OF MAXIMUM PERIL TO THE EU.

MARCH 29, 2017

British Prime Minister Theresa May, right, triggers Article 50 of the Lisbon Treaty, informing the European Union of the United Kingdom’s intention to leave the 28-nation bloc.

DECEMBER

Barnier wants divorcefirst negotiations, delaying talks on any trade deal until Britain agrees to: pay a 60 billion Euro exit bill; protect the rights of 3.3 million EU nationals living in the U.K.; settle border issues in Northern Ireland and status of Gibraltar.

Chief Brexit advocate, Nigel Farage

MARCH 2018

London’s preferred deadline for agreeing to a transition deal. The longer Britain waits, the higher the risk that businesses will move investment away from the U.K.

MARCHAPRIL

The 27 other members of the EU draw up guidelines — described as a broad-brush response — on how to handle talks.

MAYSEPTEMBER

The European Commission develops a negotiating mandate, which must then be approved by the EU-27 before negotiations with the U.K. can begin. It could take as long as six months for the EU to decide its opening position.

OCTOBER 2018

Barnier’s deadline to agree to a transition deal. This gives five months for ratification by EU member states, European Council and European Parliament.

APRIL 29

The date of a summit of the so-called EU-27, at which guidelines on talks and “red lines” that cannot be compromised are endorsed. The talks would be led by the EU’s chief negotiator, Michel Barnier, left.

MAY

In London, the Great Repeal Bill will revoke the European Communities Act, which incorporates some 80,000 EU regulations into British law “wherever practical” and ends jurisdiction of the European Court of Justice.

MARCH 2019

Ratification of the transition deal. U.K. parliament also votes on the deal.

MARCH 29, 2019

Britain leaves the European Union.