Brexit
Britain's 2016 vote to leave the European Union and the four years of negotiation that followed, the first time a member state has reversed European integration.
On 23 June 2016 the United Kingdom voted by 51.9 percent to 48.1 percent, on a turnout of 72.2 percent, to leave the European Union. It was the first time in the history of European integration that the process ran backwards. David Cameron resigned the next morning, and the country spent the following four and a half years working out what the answer to the question had actually meant.
How it happened
The referendum was called to settle an argument inside the Conservative Party. Cameron promised it in 2013 under pressure from his own backbenchers and from the UK Independence Party, expecting to win it as he had won the Scottish referendum in 2014.
Three currents met and carried the vote. Sovereignty was the intellectual case: that too many decisions binding Britain were made in Brussels by people no British voter could remove. Immigration was the emotional one, and it had a specific cause, since Britain was one of only three states to open its labor market immediately to the 2004 accession countries, producing a scale of arrival nobody had forecast, concentrated in towns with strained housing and services. And economic geography supplied the anger: the vote mapped closely onto the parts of Britain that had lost industry and then absorbed a decade of austerity after 2008.
The remain campaign's answer was overwhelmingly about economic risk, which is a weak argument against people who feel they have already lost.
Getting out
Theresa May triggered Article 50 on 29 March 2017, starting a two-year clock, and then lost her parliamentary majority in an election she had called to strengthen it. Her withdrawal agreement was rejected by the Commons three times, by the largest margins in modern parliamentary history.
The problem that broke her was Ireland. The Good Friday Agreement depends on an invisible border between Northern Ireland and the Republic. If the UK left the customs union and single market, a customs border had to exist somewhere: either across the island of Ireland, which nobody would accept, or down the Irish Sea between Northern Ireland and Great Britain, which unionists would not accept. There was no third option, and the entire negotiation kept crashing into that geometry.
Boris Johnson took the Irish Sea option, won a large majority in December 2019 on the slogan Get Brexit Done, and the UK left on 31 January 2020, with the transition period ending on 31 December. The Trade and Cooperation Agreement gave tariff-free goods trade, and nothing like frictionless trade: customs declarations, rules of origin paperwork, veterinary checks, the end of automatic professional recognition, and almost no coverage of services, which are eighty percent of the British economy. The Northern Ireland arrangements were renegotiated again as the Windsor Framework in 2023.
What it did
The Office for Budget Responsibility's standing estimate is that Brexit will reduce long-run UK productivity by about four percent relative to remaining, with trade intensity permanently lower. Small exporters have been hit hardest, since the fixed cost of customs compliance falls on them disproportionately.
The immigration outcome is the sharpest irony. Ending free movement changed where migrants came from, not how many arrived. Net migration reached record levels after 2021, driven by non-EU workers, students, and humanitarian schemes, because the sectors that had relied on European labor still needed staff and the new system let them recruit further afield.
Politically it consumed the Conservative Party that delivered it, went through four prime ministers in six years, and revived the Scottish independence argument, since Scotland voted 62 percent to remain.
The echo
The most useful thing about Brexit is what it demonstrated rather than what it decided. Deep economic integration is not a technical arrangement that quietly accumulates; it is a political choice, and it can be unmade. It also showed the asymmetry involved: a bloc of 440 million negotiating with a country of 67 million sets the terms, and the exit is far more expensive and complicated than the entry ever was.
No other member has since seriously proposed following. That, more than any clause of the Trade and Cooperation Agreement, is Brexit's largest effect on Europe.