Theresa May has triggered Article 50, and it's anyone's guess what happens next

Nine months after Britain’s historic vote to leave the European Union, the big day is finally here. British Prime Minister Theresa May triggered the formal process of removing her country from the EU Wednesday, which means that Brexit begins now – but what exactly that will mean for the U.K. is still anyone’s guess.

Wednesday marked the beginning of a two-year period of complicated negotiations on the departure, casting the country of 64 million people into the unknown. If no deal is reached on the terms of Britain’s exit and its future relationship with the bloc, the U.K.’s membership will lapse anyway on March 29, 2019.

A letter signed by May was hand-delivered to Donald Tusk, president of the European Council in Brussels Wednesday, officially notifying the bloc that Britain was triggering Article 50 of the Lisbon Treaty in order to end its 44-year membership. Tusk told reporters there is “no reason to pretend that this is a happy day, neither in Brussels nor in London. After all, most Europeans, including almost half the British voters, wish that we would stay together, not drift apart.”

As the letter was delivered, May delivered a speech to the British Parliament announcing that Brexit was underway and calling on the country to unite following the divisive vote.

“In accordance with the wishes of the people, the United Kingdom is leaving the European Union. This is a historic moment from which there can be no turning back,” she said.

“We are one great union of people and nations with a proud history and a bright future. And, now that the decision has been made to leave the EU, it is time to come together.”

May said she had a plan for “a new deep and special partnership between Britain and the European Union,” and that while the U.K. was “leaving the institutions of the European Union, we are not leaving Europe.”

The shock 52-48 vote for Brexit in last June’s referendum has left the U.K. deeply split, with many passionate Remainers – mainly concentrated in London, Scotland, and Northern Ireland – calling the move a historic blunder. One Conservative former Cabinet minister, Michael Heseltine, told the Guardian newspaper that Brexit was the “worst peacetime decision taken by any modern postwar government.”

May now faces a formidable challenge in negotiating an exit deal with the remaining 27 EU member states on issues like trade and security within a two-year timeframe, while fending off a fresh push for Scottish independence that could fracture the United Kingdom itself.

Scotland voted by 62 percent to remain in the EU, and its devolved Parliament voted Tuesday to seek a second referendum on independence in response to Brexit. The country’s First Minister, Nicola Sturgeon, tweeted Wednesday: “Today the PM will take the U.K. over a cliff with no idea of the landing place. Scotland didn’t vote for it and our voice has been ignored.”

In Northern Ireland, where a majority also voted to remain, nationalists have been calling for a public vote on leaving the U.K., and uniting with the Republic of Ireland.

The health of Britain’s $2.6 trillion economy hinges on the talks, and London will be keen to ensure its role as a key global financial center. May has said she will seek to secure the greatest possible access to European markets, but she has indicated that Brexit will inevitably mean withdrawing from the EU’s single market of 500 million people – as staying in would mean accepting the free movement of EU subjects into Britain. In its place, Britain may seek a bespoke trade deal with the EU or fall back on World Trade Organization rules if no deal is reached. European officials have warned that resolving complex trade issues will take longer than the two-year Brexit timeframe.

Another pressing issue surrounds the future rights of the approximately 3 million EU citizens living in the U.K. and the estimated 1.2 million British nationals in Europe. May told Parliament Wednesday that securing those rights would be an early priority in talks with the EU. Also likely to feature early in negotiations is the subject of the divorce bill itself, with some EU member states calling for Britain to pay a hefty sum of up to 60 billion euros ($64 billion) before the bloc starts any talks on a future trading relationship.

For the EU, already weathering the impacts of an unprecedented migration influx and a debt crisis, Britain’s departure comes as a huge blow. With populist Euroskeptic parties gaining support across the Continent, they may not be inclined to let Britain go easily for fear of encouraging further defections. A European Parliament resolution leaked to the Guardian newspaper suggested that Britain could face a tough response, with no trade deal to be granted within the two-year negotiations window and transitional arrangements to help soften the impacts of Brexit limited to three years.

“It’s bad news for everybody. It’s a wedge pushed into the European project,” French presidential candidate Emmanuel Macron told Europe 1 Radio.

But Nigel Farage, the former leader of the Ukip party often credited as the architect of Brexit, was predictably delighted. “The impossible dream is happening. Today we pass the point of no return,” he tweeted.

Jeremy Corbyn, the leader of Britain’s opposition Labour party, said his party respected the public’s vote to leave but warned that “Britain is going to change as a result.”

“The question is how,” he said. “It will be a national failure of historic proportions if the prime minister comes back from Brussels without having secured protection for jobs and living standards.”