On May 9, 1950, Robert Schuman, the French Minister for Foreign Affairs stated: “Europe will not be made all at once, or according to a single plan. It will be built through concrete achievements which first create a de facto solidarity.”Along with Jean Monnet, Schuman made a proposal (known as the Schuman Plan) to pool France's and Germany's two most important resources-- coal and steel-- together, into an organisation that would also be open to other European countries.

Robert Schuman’s proposal gave birth to a community that specialised in managing coal and steel, which was very important, but also quite limited. To further European integration, the six member countries decided to enter into economic cooperation as well. With this goal in mind, the same six countries signed the Treaty of Rome, which established the EEC (the European Economic Community), and Euratom (the European Atomic Energy Community).
The goal of the EEC was to create a large common market in Europe characterised by a customs union and the gradual removal of tariffs, so that people and goods could move freely betwen all the member countries. The EEC also sought to implement common policies in all of the member countries, particularly in the field of agriculture. The principle aim of the European Atomic Energy Community, meanwhile, was the joint development of peaceful atomic energy in Europe.

To facilitate the work of the fledgling European Community, four new institutions were established in 1958: the European Commission, a Council of Ministers, a Parliamentary Assembly (later titled the "European Parliament"), and a Court of Justice.

The first enlargement of the EEC was in 1973. The United Kingdom, Ireland and Denmark entered into the EEC, increasing the number of member states from 6 to 9. Norway rejected membership in a referendum.

After the fall of the regime of the Colonels, Greece enter in the European Community in 1981.

In 1986, the Single European Act (SEA) sets a deadline for the creation of the Single Market: January 1993. This new treaty required the member states to commit to establishing a Europe without internal borders by that date. No restrictions, whether regulations or taxes, could delay the establishment of a genuine European internal market.
Some 300 measures were taken to eliminate the various obstacles impeding the free circulation of goods and people: physical barriers, technical barriers, fiscal barriers, etc.

The Maastricht Treaty, signed on February 7, 1992, took European construction to a new level. It established the European Union (EU) and added a political dimension to the Community. It sets out clear rules to implement the future single currency, foreign and security policy as well as a closer cooperation regarding justice and domestic affairs.
Another innovation of the Maastricht Treaty was the creation of EU citizenship. From then on, every person holding the nationality of a EU country automatically became a citizen of the EU. Moreover, the EU citizenship provided rights, such as the right to move and reside freely within the EU, the right to vote for and to stand as a candidate in European Parliament and municipal elections, the right to register a complaint with the European Ombudsman, etc.

It was also in 1993 that accession negotiations started with Austria, Finland, and Sweden. These negotiations ended one year later and on January 1, 1995, the European Union welcomed three new members, making it a Europe of 15.

The Schengen Agreements enter into force in seven Member States: Germany, Belgium, Spain, France, Luxembourg, the Netherlands and Portugal. The travellers, of all nationalities, can go in these countries without identity check to the borders. Other countries since joined the Schengen zone.

The Heads of State or Government agreed on a new treaty - the Treaty of Nice - during the European Council in Nice on December 7-11, 2000. This treaty was signed on February 26, 2001, by 15 countries. Its primary aim was to change the institutional and decision-making processes of the European Union, to allow for the enlargement to 25 Member States. The Treaty of Nice became effective on February 1, 2003.

After a long process of negotiations between the Union and individual countries, ten new countries joined the European Union: Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, the Czech Republic, Slovakia, and Slovenia.

On October, 29^th 2004, the 25 EU countries signed a new treaty: the Treaty Establishing a Constitution of Europe. This text offered a series of measures to make the Union more transparent, more efficient, and closer to its citizens. It aims at better organising the decision-making and management process in all the 25 EU countries, and even more. The Treaty was then rejected by France and the Netherlands in 2005. All EU State Members had to ratify the treaty though. Following the rejection, EU leaders started to consider reforming the EU treaties and its future.

On July 1, 2013, Croatia became the most recent country to join the European Union, making it a Union of 28.

On June 23 2016, a referendum was held in the UK to decide whether it should leave or remain in the European Union. Leave won by 51.9%. The UK invoked Article 50 of the Treaty on European Union (TEU) which began the member state's withdrawal from the EU. British political leaders launched the process of leaving the EU on March 29 2017.
From this date, the EU and the UK have two years to negotiate and reach a withdrawal agreement.

The UK should have left the EU on March 29 2019. Upon the UK’s proposal, the European Council decided to grant an additional period of negotiation, during which the withdrawal agreement get by Theresa May in November 2018 should be ratified. On April 10 2019, the 27 EU leaders agreed to postpone Brexit on October 31 2019, for the latest. However, the UK could have left before this date if the withdrawal agreement was ratified by the UK and the EU. To make a long story short, the UK is still an EU Member State and will vote in European elections, just like other EU Member States.

The UK has left the EU on 31 January 2020, thus the transition period stared .

On 27 April 2021 the European Parliament gave final approval to the EU-UK Trade and Cooperation Agreement. On 29 April 2021 the European Council approved the conclusion of the agreement by way of a Council Decision. As a result, the agreements between the UK and the EU came into force on 1 May 2021. This was the last official step in the formalising the new relationship between the EU and the UK.

Nine countries are currently candidates for EU membership: Albania, Bosnia and Herzegovina (since December 2022), North Macedonia, Moldova (since June 2022), Montenegro, Serbia, Turkey, Ukraine (since June 2022), and Georgia (since December 2023).

Another country, Kosovo, is considered a "potential candidate country" by the European Council.

The negotiation process is conducted individually based on a negotiation framework defined by the European Council on the proposal of the Commission.

EU membership becomes effective once the negotiations are concluded and the accession agreements have been ratified by all EU member states as well as by the candidate country.