Article 50 of the Treaty of Lisbon, signed by member nations back in 2007 and effective in 2009, requires the EU to negotiate a severance with any nation that gives at least two years notice that it intends to leave [source: Lisbon Treaty].
So far, though, only one nation has opted to do that. In June 2016, the U.K. held a referendum on whether it should invoke Article 50, though most people knew it as Brexit. Though polls had predicted otherwise, U.K. voters shocked Europe by voting 52-48 percent to leave the EU [source: Archick].
Advocates of Brexit argued that the EU was a bad deal for the British, because it allowed workers from elsewhere in Europe to migrate to the U.K. and drive wages there down. They also felt that the EU had too much say over the affairs of the U.K. [source: Lee].
Others complained that the EU's regulations for fighting climate change and increasing renewable energy use were costly burdens to the U.K. economy. One thinktank calculated that the annual cost of fulfilling the 100 "most burdensome" EU regulations was 33.3 billion pounds (U.S. $42.6 billion) [source: BBC News].
The Brexit decision had all sorts of consequences and complications, many of which voters probably didn't foresee. For instance, the governor of the bank of England said in May 2017 the U.K. had already lost 2 percent of predicted growth and 4 percent from household incomes since the referendum [source: The Guardian]. And the thinktank's calculation, others noted, didn't account for the annual 58.6 billion pounds ($75.12 billion) in benefits Britain gained from being in the EU [source: BBC News].
On the other hand in December 2017, the firm EY predicted the U.K. would lose 10,500 financial industry jobs on day one of Brexit, as banks, brokerages and other financial companies started moving their operations to the continent to ensure that they'd still have unfettered access to European customers. But the City of London Corp. downgraded that loss to just 5,000 in August 2018 [sources: Musaddique, Contiguglia]. The real number, of course, is still a mystery.
Another unsettled matter is what happens to the border between Northern Ireland (part of the U.K.) and the Republic of Ireland, which is remaining in the EU. The 1998 Belfast Agreement, which put an end to the fighting in Northern Ireland, also did away with border checks. Neither the U.K. nor Ireland wants a return to border checks, but as of late 2018, they hadn't figured out a way to resolve this [source: Carswell].