The British government announced Monday that it will formally begin its exit from the EU on March 29. Prime Minister Theresa May will invoke Article 50 of the key EU treaty, the official start of the two-year divorce process.
Then comes the hard part- the arguments, the lawyers, the squabbles over money. Here's a look at the main issues and what happens next:
The EU 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 eurosceptic politicians and journalists regularly railing against regulations imposed by EU headquarters in Brussels.
Former Prime Minister David Cameron offered voters a referendum on EU membership, and in June they voted by 52-48% to leave.
The British government will invoke Article 50 of the EU's Lisbon Treaty, which says a member state may "notify the European Council of its intention" to leave the bloc.
The Department for Exiting the EU says notification will come in a letter from May to European Council President Donald Tusk. May will also announce the news in 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 EU member.
The timing of Article 50 was up to Britain. What happens next is up to the EU.
European Commission spokesman Margaritis Schinas said Monday that the EU is "ready to begin negotiations."
Tusk has said that that once EU 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 in April or May to finalise their negotiating platform.
On the British side, Davis will take the lead, reporting to May. Britain's ambassador to the EU, 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 EU side, it's complicated. As Britain's Institute for Government recently pointed out, "the UK 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.
Britain's vote to leave the EU has meant uncertainty for 3m EU citizens living in the UK, and 1m Britons who reside in the 27 other nations of the bloc. Both sides agree that giving such citizens a guarantee that they will be able to stay where they are is a top priority.
The first major battle is likely to be about money. The EU says Britain must pay a hefty divorce bill of up to $64bn, to cover EU staff pensions and other expenses the UK 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 maintain free trade in goods and services with the bloc, without accepting the EU's core principle of free movement of workers. Britain has said it will impose limits on immigration, and so will have to leave the EU's single market and customs union. That makes some barriers to trade seem inevitable.
Under the terms of Article 50, Britain will cease to be an EU member in March 2019.
But EU negotiators warn it could take two years just to settle the divorce terms; agreeing on a new relationship for the UK and the EU could take years longer. If the rest of the EU agrees, the two-year negotiating period can be extended, leaving Britain in the EU for a while longer. 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 the talks are going nowhere.
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 EU John Kerr, who wrote Article 50, says "it is not irrevocable. You can change your mind while the process is going on."
However, domestic political pressures make it unlikely that the British government would try a U-turn. May will probably take her cue from a catchphrase of predecessor Margaret Thatcher: "The lady's not for turning."