The European Union begins with this, the founding treaty of the Western Union between 1948 and 1954, at which point it is amended as the Modified Treaty of Brussels and serves as the founding treaty of the Western European Union (until its termination in 2010).

In the first enlargement of the European Communities (EC), Denmark, Ireland and the United Kingdom (UK) accede to the EC on 1 January 1973. For the British, this is seen as a victory for Conservative Prime Minister Ted Heath. This entry of the UK to the EU is put to a referendum two years later and is supported by 67% of those that voted.

As promised in the Labour Party manifesto for the October 1974 general election, this referendum allows British voters to decide whether to remain in the EC. It is the first UK national referendum and leads to the electorate voting by 67% in favour of remaining in the EC on a turnout of 64%.

Labour pledged to renegotiate the terms of British accession to the EC and then to consult the public on whether Britain should stay in the EC on the new terms. The EC heads of government agreed to a deal earlier in 1975 and the government recommended a vote in favour of continued membership based on this deal.

The first major revision of the 1957 Treaty of Rome, this Amending Treaty (or Single European Act) sets the European Community an objective of establishing a single market and codified European Political Cooperation (the forerunner of the EU's Common Foreign and Security Policy).

Otherwise known as the Maastricht Treaty as it is originally signed on 7 February 1992 by the members of the European Communities in Maastricht, Netherlands, this leads directly to further European integration. The treaty founds the European Union and establishes its pillar structure which stays in place until the Lisbon Treaty comes into force. The treaty also greatly expands the competencies of the EEC/EU and leads to the creation of the single European currency, the euro.

The Treaty of Lisbon amends the two treaties which formed the constitutional basis of the European Union (the Treaty of Rome and the Treaty of European Union).

The Treaty of Lisbon moves the EU to majority voting, a more powerful Europan Parliament and creates a consolidated legal personality for the EU, a President of the European Council and a High Representative of the Union for Foreign Affairs and Security Policy. It is signed by EU member states and also enters into force on 1 December 2009.

This constitutes a severe worldwide economic crisis, considered by many economists to be the most serious financial shock since the Great Depression of the 1930s.

It begins in 2007 with a crisis in the subprime mortgage market in the United States and then develops into a full-blown international banking crisis with the collapse of the investment bank Lehman Brothers on 15 September 2008. Excessive risk-taking by banks such as Lehman Brothers helps to magnify the financial impact globally. Massive bailouts of financial institutions and other monetary and fiscal policies are subsequently employed to prevent a possible collapse of the entire world financial system.

The crisis is followed by the global Great Recession.

This begins when eurozone members Greece, Portugal, Ireland, Spain and Cyprus discover they cannot repay or refinance their government debt or bail out their banks. Causes of the crisis include sovereign debts rising uncontrollably as banks are bailed out and adjustments to the structure of the eurozone currency union without a fiscal union behind it.

The crisis carries over into 2010, with European nations implementing financial support measures such as the European Financial Stability Facility (EFSF) and European Stability Mechanism (ESM). The ECB also responds by lowering interest rates and providing emergency loans of more than one trillion euros to maintain money flows between European banks.

In 2012, the ECB responds to international pressure and calms financial markets by announcing free unlimited support for all eurozone countries in trouble. Stability and growth returns. Ireland and Portugal exit their bailout programmes in 2014, and Greece and Cyprus partly regain market access in the same year.

The crisis damages national economies and labour markets throughout Europe, with unemployment rates in Greece and Spain reaching 27% and has a major political impact on the ruling governments in 10 out of 19 eurozone countries, contributing to power shifts in Greece, Ireland, France, Italy, Portugal, Spain, Slovenia, Slovakia, Belgium and the Netherlands, as well as in the United Kingdom.

The most successful party overall in the United Kingdom European parliament elections of 2014 are the UK Independence Party (UKIP) which wins 24 seats and 27% of the popular vote on a challenging manifesto. It is the first time a political party other than the Labour Party or Conservative Party has won the popular vote at a British election since the 1906 general election.

This is also the first time a party other than Labour or the Conservative Party has won the largest number of seats in a national election since the December 1910 general election, while the 23.1% of the vote won by the Conservatives is the lowest vote-share for the party in a national election (until the 2019 EU elections). The governing Conservative Party is also pushed into third place for the first time at a European Parliament election.

Between 2015 and 2016 more than one million asylum seekers and economic migrants arrive in the EU from across the Mediterranean Sea or via Southeast Europe, following Turkey's migrant crisis. Research suggests that record population growth in Africa and the Middle East is one of the drivers of the crisis.

​The majority of migrants are Muslim (mainly Sunni) and the United Nations High Commissioner for Refugees suggest the top migrant nationalities between 2015 and 2016 are Syrian, Afghan and Iraqi.

​The number of people crossing EU borders illegally then falls to just over 200,000 in 2017, by which time the speed and volume of migrants entering the EU during the crisis have caused significant political controversy.

Polls and commentators predicted that the outcome of this election would be too close to call and would result in a second hung parliament, similar to the 2010 election. The Conservative Party then unexpectedly wins an outright majority, which bears a resemblance to its victory at the 1992 general election, winning 330 seats and 36.9% of the vote, as well as a working majority of twelve seats.

The Conservative majority means that Cameron can fulfil a manifesto commitment to renegotiate British membership of the European Union. This renegotiation proves largely fruitless and fails to have a positive impact on the UK European Union membership referendum in 2016 (also promised in the manifesto).

Despite the UK government's recommendation that the country remains a member of the EU, in the UK European Union membership referendum of 23 June 2016, 51.9% of voters choose for the United Kingdom to leave the EU.

On 31 January 2020, the UK becomes the first member state to ever leave the European Union after the signing of a trade deal between the European Union and the United Kingdom.