LONDON (AP) — Britain officially leaves the European Union on Friday after a debilitating political period that has bitterly divided the nation since the 2016 Brexit referendum.

Difficult negotiations setting out the new relationship between Britain and its European neighbors will continue throughout 2020.

This series of stories chronicles Britain’s tortured relationship with Europe from the post-World War II years to the present.

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After a modest renegotiation of Britain’s membership in the European Union, Prime Minister David Cameron set the date for a referendum for June 23, 2016. He would back and lead the “remain” campaign, but he gave his ministers the right to back “leave” and many of them did.

Though some big names came out in favour of leaving the EU, such as former London Mayor Boris Johnson, the prevailing view as the referendum campaign began in earnest was that the “remain” camp would win and confirm Britain’s membership in the bloc for at least another generation.

There would, the consensus went, be a rush to the status quo when the time actually came, in much the same way there had been in Scotland two years earlier when it voted, by a convincing 10-point majority, to remain part of the United Kingdom in an independence referendum.

A dizzying array of forecasts of an imminent recession should Britain vote to leave were bandied about, but nothing appeared to stick. The “leave” camp led in many opinion polls during the campaign, its message of “Take Back Control” seemingly resonating far and wide, and beyond any material considerations.

Promises of much more money for the financially strained National Health Service on a side of a bus also caught the mood of a country tired of years of austerity. As did, it must be said, warnings about untrammelled immigration at a time when Europe was struggling to cope with an unprecedented surge of migrants, largely from war-torn Syria.

Brexit wouldn’t be a problem, according to the likes of Johnson, who claimed that Britain could “have our cake and eat it.”

The whole campaign came to a halt a week before the referendum when Labour lawmaker and big remain backer Jo Cox was killed by a far-right extremist. When campaigning resumed a few days later, it was widely thought that the remain camp would win partly because of the horror associated with the killing of Cox.

On the day of the referendum itself, British bookmakers thought the vote was in the bag for remain as they offered odds of 10-to-1 on. The pound also pushed sharply higher on foreign exchange markets.

But when the first results started coming in just before midnight from many provincial towns, it was clear that something very different was taking place. Though Scotland, London and most of the other big cities voted, often by big margins, to remain in the EU, whole swathes of the country backed leave.

The pound tanked around 15% before dawn had broken on June 24 and David Cameron announced his resignation soon after.

“Let June 23 go down in our history as our Independence Day,” UK Independence Party leader Nigel Farage roared to cheering Brexit supporters.

Brexit had won, but Brexit wasn’t done.

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Follow AP’s full coverage of Brexit and British politics at https://www.apnews.com/Brexit