The Treaty of Maastricht
The Lisbon Treaty.
The Maastricht Treaty of 1992 accomplished changing the EC into the EU, but did NOT accomplish Political unity of all nations in Europe.
The former came into being by the Treaty of Rome 1957 and the latter by the Treaty on European Union of 1992 but the Treaty of Lisbon of 2007 (ratified in 2009) amended those two treaties to make the EC (renamed TFEU, Treaty on the Functioning of the EU) as one of the three pillars of the EU.
Maastricht Treaty
The Treaty of Brussels.
The Treaty of the European Union, or the Maastricht Treaty
They are a member of the EU and the Schengen Treaty.
The Treaty on the European Union or the Maastricht Treaty
The Lisbon Treaty.
The Maastricht Treaty.
Not with the whole EU but only with some of the EU Member States.
No treaty, but a referendum on whether or not to join the EEC was held in the UK.