Britain on Friday will formally leave the European Union, ending an intense nearly four years of political fighting over Brexit since Brits voted to leave the bloc that has consumed British politics and saw the ouster of two prime ministers.

Britain will formally leave at 11 p.m. U.K. time (6 p.m. ET) and Prime Minister Boris Johnson is expected to address the public about an hour before the departure time. Elsewhere throughout the country, the day will be marked by celebrations and some mourning from those opposed to the move. But in typically British fashion, the celebrations are expected to be muted.

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Some of those celebrations have already occurred. MEP Nigel Farage and fellow members of the Brexit Party waved British flags on Wednesday in the E.U. Parliament chamber, while pro-Remain MEPs were seen mourning the coming departure.

The 2016 vote to leave the E.U. in a one-off referendum sent shockwaves throughout the globe and is frequently cited as a groundbreaking populist moment that previewed President Trump's victory months later in the U.S. general election.

But while Trump took the White House at the beginning of 2017, "Leave" voters hoping to see their votes put into action were to be made to wait a lot longer for the result to be delivered.

On the morning after the historic vote, then-Prime Minister David Cameron announced he would step down. He was replaced by fellow Tory Theresa May. May struggled throughout 2017 and early 2018 as Brexit ate up the political oxygen for months and lawmakers mulled over what form Brexit should take.

The heart of the matter then became about how to leave. May’s team started negotiating a formal withdrawal agreement with E.U. leaders. But she was faced with pushback both from E.U. leaders, who wanted as “soft” a Brexit as possible, as well as members of her own party who wanted a harder Brexit, possibly by leaving with no deal at all.

She eventually hashed out a deal with E.U. leaders, but it was extremely unpopular at home, including with many in her own party.

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The key issue was the backstop -- a mechanism designed to stop a hard border between Ireland and Northern Ireland. While intended as a safety net in which the U.K. would remain in a customs union until a trade deal was secured, Brexiteers pointed to the lack of a unilateral exit mechanism as evidence that it could lead to Britain never leaving at all.

In addition, a growing coalition of moderate Tories and left-wing politicians began to increase their calls for a second referendum (which they said was not a do-over, but would be a vote on whatever deal came from Brussels). Many of them eventually said that their aim was to stop Brexit altogether. May’s woes were exacerbated by the 2017 general election that had seen her returned to 10 Downing Street, but with a narrower majority after she barely clung to power.

The consequence in 2018 and early 2019 was a parliamentary stalemate. May’s deal was shot down multiple times in the House of Commons as she kept trying to push it over the finish line as she repeatedly extended the Brexit deadline. But with a slim majority and a disunited party -- there was no chance, even as she tried to repackage it.

The pressure mounted from both within and outside her party, and in the month of May, she announced her departure. She was in turn replaced by former Foreign Secretary Boris Johnson -- who had resigned a year before in protest of May’s deal, which he had branded a surrender to Brussels.

Johnson brought a harder line to Brexit, having been one of the main campaigners for Britain’s departure in 2016. (May had backed Remain.) Once in power, Johnson found he had similar troubles to May in the form of the now-evaporated majority in Parliament.

His first job was to renegotiate the deal, particularly the backstop. When he returned from Europe with those changes to the backstop in hand -- which satisfied the Brexiteer wing of the party -- he then turned his efforts to expanding his majority via a general election. But that was opposed by the opposition Labour Party until after a no-deal exit could be ruled out at the then-deadline of October.

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Once that passed, and the deadline extended to Jan. 31, Labour agreed to an election, one that was summarized simply by U.K. media outlets as “The Brexit Election.” While other issues such as the country’s national health service were also on the ballot, Brexit dominated much of the conversation.

Johnson’s plea to the voters was simple: “Get Brexit done.”

It was a plea that worked. Johnson was returned with an overwhelming majority, way beyond what even optimistic members of his party had hoped for. It made his withdrawal agreement certain to pass, and therefore Brexit itself was then a certainty.

But as much as Brexit is over, there is still more Brexit left to figure out. British and European negotiators will now go into negotiations for the resultant trade deal. Johnson has promised to get that done by the end of 2020, but E.U. forces have suggested that that may not be possible.

Johnson, meanwhile, will also be looking to Washington for a U.S.-U.K. trade deal, one Trump has repeatedly promised and will be vital for a strong post-Brexit future for the U.K.

Secretary of State Mike Pompeo offered his support for America's ally across the pond after a visit Thursday.

"The U.K. is a critical ally, and we’ll stand with them all the way. #SideBySide," he tweeted.

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There is also the question of what will happen to the E.U. Having lost one of its largest contributors, it will be closely watched as to how it handles the fallout of Brexit, and whether it engages in political reforms to prevent other nations from exiting.

So far there are no significant indications that other nations will depart the bloc, but if Britain’s departure leads to greater prosperity and independence there, it could quickly lead to anti-E.U. movements gaining momentum in other E.U. member nations.