State governments must agree unanimously in the Council for the union to adopt some policies; for others, collective decisions are made by qualified majority voting.

In the 1950s, six core states founded the EU's predecessor European Communities (Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany).

No other member state has withdrawn from the EU and none has been suspended, although some dependent territories or semi-autonomous areas have left.

Furthermore, it has to be willing to accept all the obligations of membership, such as adopting all previously agreed law (the 170,000 pages of acquis communautaire) and switching to the euro.

Introduced in the Treaty of Amsterdam, Article 7 outlines that if a member persistently breaches the EU's founding principles (liberty, democracy, human rights and so forth, outlined in TEU Article 2) then the European Council can vote to suspend any rights of membership, such as voting and representation.

[21] The state in question would still be bound by the obligations treaties and the Council acting by majority may alter or lift such sanctions.

The Treaty of Nice included a preventive mechanism whereby the council, acting by majority, may identify a potential breach and make recommendations to the state to rectify it before action is taken against it as outlined above.

The Lisbon Treaty changed this and included the first provision and procedure of a member state to leave the bloc.

[23] After an extended period of negotiation and internal political debate the UK eventually withdrew from the EU on 31 January 2020.

However, French Algeria, Greenland and Saint-Barthélemy did cease being part of the EU (or its predecessor) in 1962, 1985, and 2012, respectively, due to status changes.

[26] Beyond the formal withdrawal of a member state, there are a number of independence movements such as Catalonia or Flanders which could result in a similar situation to Greenland.

Were a territory of a member state to secede but wish to remain in the EU, some scholars claim it would need to reapply to join as if it were a new country applying from scratch.

[35] While the member states are sovereign, the union partially follows a supranational system for those functions agreed by treaty to be shared.

Very early on in the history of the EU, the unique state of its establishment and pooling of sovereignty was emphasised by the Court of Justice:[37] By creating a Community of unlimited duration, having its own institutions, its own personality, its own legal capacity and capacity of representation on the international plane and, more particularly, real powers stemming from a limitation of sovereignty or a transfer of powers from the States to Community, the Member States have limited their sovereign rights and have thus created a body of law which binds both their nationals and themselves...The transfer by the States from their domestic legal system to the Community legal system of the rights and obligations arising under the Treaty carries with it a permanent limitation of their sovereign rights.

The table below shows which aspects of governance are exclusively for collective action (through the commission) and which are shared to a greater or lesser extent.

As a result of the Greek government-debt crisis, Greece accepted a large austerity plan including privatisations and a sell off of state assets in exchange for their bailout.

Some, including the President of the Euro Group Jean-Claude Juncker, stated that "the sovereignty of Greece will be massively limited.

More than half of member states—16 out of 27—are parliamentary republics, while six states are constitutional monarchies, meaning they have a monarch although political powers are exercised by elected politicians.

Most republics and all the monarchies operate a parliamentary system whereby the head of state (president or monarch) has a largely ceremonial role with reserve powers.

Upper houses are composed differently in different member states: it can be directly elected like the Polish senate; indirectly elected, for example, by regional legislatures like the Federal Council of Austria; or unelected, but representing certain interest groups like the National Council of Slovenia.

Spain and Italy have systems of devolution where regions have autonomy, but the national government retains the legal right to revoke it.