On 23 June 2016, the United Kingdom voted to leave the European Union (EU) by 51.9% to 48.1% in a nationwide referendum. The result, on a turnout of 72.2%, overturned the 1975 decision to remain in the bloc and triggered a period of immense political turmoil.

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On 23 January 2013, David Cameron as prime minister delivered a landmark speech at the Bloomberg headquarters in London, committing a future Conservative government to holding an “in/out” referendum on Britain’s membership of the EU. Politically, the speech was characterised as an attempt by Cameron to manage internal pressure from eurosceptic Conservative backbenchers and external electoral pressure from the UK Independence Party (Ukip).

The Conservative manifesto for the 2015 general election stated: “The EU needs to change. And it is time for the British people – not politicians – to have their say. Only the Conservative Party will deliver real change and real choice on Europe, with an in-out referendum by the end of 2017.”

Cameron’s party defied the pre-election polls and secured a slim majority, winning 330 seats (up 24) on a 36.9% vote share.

Following a limited renegotiation with the EU, Cameron named the date for the vote according to the European Union Referendum Act 2015. At this point, the prime minister had already triumphed in two referendums on Scottish independence (2014) and electoral reform (2011).

On 5 January 2016, Cameron confirmed that government ministers would be free to campaign for either side of the referendum. The prime minister’s statement, delivered to the commons, was widely seen as the unofficial starting gun for the referendum campaign.

Cameron officially declared the government’s formal pro-Remain position on 20 February. “The choice is in your hands”, he stated. “But my recommendation is clear. I believe that Britain will be safer, stronger and better off by remaining in a reformed European Union.”

It came as six cabinet ministers broke ranks with Cameron to join the official pro-Brexit campaign. The “gang of six” included Michael Gove, the justice secretary; Chris Grayling, the commons leader; Iain Duncan Smith, the work and pensions secretary; John Whittingdale, the culture secretary; Theresa Villiers, the Northern Ireland secretary; and Priti Patel, the minister of state for employment (who attended cabinet).

Gove, a close political ally of Cameron, released a 1,500-word essay outlining what he called “the most difficult decision of my political life.”

Boris Johnson announced that he would campaign for Britain to leave the EU on 21 February. Johnson, coming to the end of his second term as mayor of London, soon assumed the position as de facto leader of the Leave campaign.

Announcing his decision outside his home in London, Johnson said that “after a huge amount of heartache” he had concluded that the EU was “in danger of getting out of proper democratic control”. It later emerged that Johnson had written two separate Telegraph columns, one arguing to remain and one to leave, before making his final choice.

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The campaign officially began on 15 April.

The pro-Remain Britain Stronger in Europe campaign, supported by Cameron and the government, focused on the economic risks of leaving, warning of damage to trade, jobs, and household incomes.

George Osborne, the chancellor of the exchequer, claimed that a vote to leave the EU would see the government tear up its manifesto pledges to raise taxes in an emergency budget. “Far from freeing up money to spend on public services as the leave campaign would like you to believe, quitting the EU would mean less money”, Osborne declared. “Billions less. It’s a lose-lose situation for British families and we shouldn’t risk it.”

Osborne’s claim was supported by his Labour predecessor as chancellor, Alistair Darling.

On 22 April, US president Barack Obama appeared to bolster the anti-Brexit campaign when he warned that a vote to leave would place Britain at the “back of the queue” for any future trade deal with the United States. The comment was delivered during a joint press conference with Cameron at the Foreign Office in London.

The intervention immediately provoked a furious backlash from Leave supporters. Boris Johnson, then the mayor of London and a leading figure in the Vote Leave campaign, hit back. Writing in the Sun newspaper, Johnson blamed the US president’s remarks on his “ancestral dislike of the British empire”.

Pro-Brexit figures famously derided the Remain camp’s economic warnings as “project fear”.

The Vote Leave campaign, fronted by Johnson and Gove, organised its strategy around the slogan “Take Back Control” – arguing that leaving the EU was a chance to restore parliamentary sovereignty and curb immigration. This strategy, masterminded by Dominic Cummings as Vote Leave director, capitalised on the loss aversion of the electorate.

Arguably, Vote Leave’s most potent visual weapon was a red battle bus emblazoned with the slogan: “We send the EU £350 million a week – let’s fund our NHS instead.” While independent authorities rubbished the accuracy of this £350 million figure, the messaging effectively linked the abstract notion of sovereignty to public service funding. It also kept the figure at the centre of public debate despite disputes over its precise size.

In a similar vein, the pro-Remain campaign cited official HM Treasury analysis, which warned that the UK would be worse off by £4,300 per household per year in the event of a vote to leave the European Union. However, this claim failed to cut through with anything approaching the same force as the £350 million figure.

Vote Leave also capitalised on anti-immigration sentiment by explicitly leaning into the prospect of Turkey joining the European Union. Campaign leaflets claimed that “Turkey (population 76 million) is joining the EU. Vote Leave, take back control.”

This claim, delivered forcefully by pro-Brexit defence minister Penny Mordaunt, was directly contradicted by Cameron. Mordaunt suggested that Britain’s veto would not, in practice, prevent Turkey’s accession. The prime minister dismissed this argument as “very misleading”, adding: “Britain and every other country in the EU has a veto on another country joining. That is a fact.”

The anti-establishment, more vibes-based tenor of the pro-Brexit campaign was encapsulated in a memorable intervention by Michael Gove, who remarked in an interview with Sky News that “people in this country have had enough of experts”.

But behind the scenes, Vote Leave pioneered a highly sophisticated, data-driven campaign strategy. Under Cummings’ direction, Vote Leave spent a reported £2.7 million of its £7 million budget on the services of a Canadian digital marketing firm, Aggregate IQ. “We couldn’t have done it without them”, Cummings later said of Aggregate IQ.

Nigel Farage, the Ukip leader, was technically segregated from the official Vote Leave campaign. But Ukip nonetheless played a significant role in the referendum campaign, as a pro-Leave outrider that could appeal to voters with messaging Cummings was reluctant to embrace. Ukip’s “Breaking Point” poster, which depicted Syrian refugees near the Croatia-Slovenia border in 2015, drew condemnation from both the official Britain Stronger in Europe and Vote Leave campaigns.

On 16 June, Labour MP Jo Cox was murdered while carrying out her constituency duties in the town of Birstall, West Yorkshire. Cox’s death, a politically motivated assassination, led to an immediate suspension of campaigning by both sides in the referendum. The commons was recalled on 20 June 2016 so MPs could pay tribute to Cox, and campaigning resumed on 19 June.

On 21 June, just two days before the national vote, the BBC broadcasted its final, climactic television debate of the referendum campaign in front of a 6,000-strong audience at Wembley Arena. The EU Referendum: The Great Debate programme was hosted by veteran broadcaster David Dimbleby, and featured clashes over immigration, the economy and sovereignty.

During the debate, Sadiq Khan accused Johnson, his predecessor as London mayor, of unleashing “project hate” and peddling falsehoods, particularly over the prospect of Turkey joining the EU – which the London mayor referred to as a “big fat lie”.

Johnson, meanwhile, called the EU a “job destroying engine”.

“You can see it all across southern Europe and you can see it, alas, in this country as well”, he said, accusing the EU of imposing a “multitude of regulations” on British businesses.

Ruth Davidson, the pro-Remain Scottish Conservative leader, won cross-party praise for her performance, including from the deputy Labour leader, Tom Watson.

In his closing statement, Johnson declared: “At the end of this campaign I think you’ll agree there is a very clear choice between those on their side who speak of nothing but fear of the consequences of leaving the EU, and we on our side who offer hope.”

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Britain, in the end, backed Brexit. But the result divided the four nations of the United Kingdom. England voted to leave by 53.4%, and Wales by 52.5%. However, Scotland voted 62% to remain, and Northern Ireland by 55.8%.

A full 95.9% of voters in Gibraltar opted to remain in the EU.

In the immediate aftermath of the result, the value of the pound plummeted on financial markets. Mark Carney, the governor of the Bank of England, announced £250 billion in extra funds to support financial markets.

Later that morning, Cameron announced his intention to resign as prime minister.

In his final speech outside 10 Downing Street, Cameron stated: “The British people have made a very clear decision to take a different path and as such I think the country requires fresh leadership to take it in this direction.”

The 2016 Conservative leadership campaign continued the relative chaos of the referendum campaign. Johnson, the apparent frontrunner, was summarily betrayed by Gove. The former London mayor chose not to stand; Gove, meanwhile, failed to secure the support of Conservative MPs.

Theresa May, the pro-Remain home secretary, later became the leader of the Conservative Party and prime minister following the sudden withdrawal of her rival, Andrea Leadsom.

The referendum result was similarly traumatic for the Labour Party. In a coup organised by the shadow foreign secretary, Hilary Benn, the party’s frontbench turned on Jeremy Corbyn. The incumbent Labour leader, a veteran eurosceptic, was accused of mounting an unenthusiastic defence of the European project.

During the campaign, Corbyn remained a relatively marginal figure. On one occasion, he acknowledged his limited enthusiasm for the EU, conceding that his support for Remain stood at “seven, or seven-and-a-half, out of 10.”

On 28 June, Corbyn suffered an emphatic defeat in an unofficial “no-confidence” vote among his own MPs, with 172 MPs voting against him and only 40 offering their support. In the subsequent election, held over the summer, Corbyn defeated Owen Smith comfortably.

Smith, a former frontbencher, won just 38.2% of the members’ vote.