LONDON --The United Kingdom filed for divorce from the European Union on Wednesday, overturning four decades of integration with its neighbors, demolishing the notion that EU expansion is inevitable and shaking the foundations of a bloc that is facing challenges to its identity and its place in the world.
Britain's top envoy to the EU, Tim Barrow, hand-delivered a letter to European Council President Donald Tusk formally triggering a two-year countdown to the final split.
"Today the government acts on the democratic will of the British people," Prime Minister Theresa May told lawmakers in the House of Commons, adding, "This is an historic moment from which there can be no turning back."
There is "no reason to pretend this is a happy day," Tusk told reporters, emphasizing that the priority now is to minimize costs for EU citizens and member states.
To Britain, he said: "We already miss you."
But for Britons who voted 52 to 48 percent to leave the bloc in a referendum nine months ago, it was a time for celebration.
"I voted for Brexit and today is the day that vote starts to count," said Charles Goodacre, a former taxi driver, in the northern England city of Sunderland. "Things have been bad round here for a while and we needed a change."
Former U.K. Independence Party leader Nigel Farage, who campaigned for years to take Brexit from fringe cause to reality, said Britain had passed "the point of no return."
"I can still, to be honest with you, scarcely believe today has come," he said.
Others worried Britain was taking a leap in the dark.
"No one knows. No one knows how this is going to go," said Nicola Gibson, who works in London's financial district. "It's a gamble, it's a risk. It's a bit like Trump. It's such an uncertain place at the moment."
Those who campaigned for Britain to stay in the EU say they will now fight for a divorce settlement that preserves what they see as key benefits of EU membership, including free trade in goods and services and the right to live and work anywhere in the bloc.
May's six-page letter to Tusk was polite and conciliatory, stressing that Britons want to remain "committed partners and allies to our friends across the continent."
She said the two sides should "engage with one another constructively and respectfully, in a spirit of sincere cooperation."
But there was a hint of steel in May's assertion that without a good deal, "our cooperation in the fight against crime and terrorism would be weakened." That could be seen by some in Europe as a threat to withdraw British security cooperation if the U.K. does not get its way.
The loss of a major member is destabilizing for the EU, which is battling to contain a tide of nationalist and populist sentiment and faces unprecedented antipathy from the new resident of the White House.
It is even more tumultuous for Britain. For all the U.K. government's confident talk of forging a close and friendly new relationship with its neighbors, it cannot be sure whether businesses will freely be able to trade, students to study abroad or pensioners to retire with ease in other EU states. Those things have become part of life since the U.K. joined what was then called the European Economic Community in 1973.
The trigger for all the economic and constitutional uncertainty is Article 50, a previously obscure clause of the EU's Lisbon Treaty that allows a member state to withdraw.
The two sides now have until March 2019 to agree on a divorce settlement and - if possible - establish a new relationship between Britain, the world's fifth-largest economy, and the EU, a vast single market taking in half a billion people.
Tusk said he will respond by Friday with draft negotiating guidelines for the remaining 27 member states to consider. They'll meet April 29 to finalize their platform.
Talks between the EU's chief negotiator, French diplomat Michel Barnier, and his British counterpart, Brexit Secretary David Davis, are likely to start in the second half of May.
As in many divorces, the first area of conflict is likely to be money. The EU wants Britain to pay a hefty bill - Jean-Claude Juncker, president of the EU's executive Commission, put it at around 50 billion euros ($63 billion) - to cover pension liabilities for EU staff and other commitments the U.K. has agreed to.
Britain acknowledges it will have to pay something, but is sure to quibble over the size of the tab.
Negotiations will also soon hit a major contraction: Britain wants "frictionless" free trade, but says it will restore control of immigration, ending the right of EU citizens to live and work in Britain. The EU says Britain can't have full access to the single market if it doesn't accept free movement, one of the bloc's key principles.
Both Britain and the EU say a top priority will be guaranteeing the rights of 3 million EU citizens living in Britain, and 1 million Britons living elsewhere in the bloc.
The two sides also appear to disagree on how the talks will unfold. EU officials say the divorce terms must be settled before negotiators can turn to the U.K.'s future relationship with the bloc, while Britain wants the two things discussed simultaneously.
The U.K. has raised the prospect it could walk away without a deal if talks falter, though May said in her letter that both sides should "work hard to avoid that outcome."
Brexit has profound implications for Britain's economy, society and even unity. The divisive decision to leave the EU has given new impetus to the drive for Scottish independence, and undermined the foundations of Northern Ireland's peace settlement.
It's also a major blow to the EU, after decades of expansion, to lose one of its largest members. Anti-EU populists including French far-right leader Marine Le Pen hope the impulses that drove Britain to turn its back on the EU will be repeated across the continent.
Many Britons who voted to leave were seeking to regain control of migration. That is what worries some on the other side, because many British businesses rely on European workers.
Piotr Wierzbicki, a Polish engineer flying to London from Warsaw airport, said the British "shot themselves in the foot" by voting to leave the EU, and predicted it would be harder for Europeans to travel to and from the U.K.
"It will be bad for their economy and it will be bad for the EU," he said.