From the beginning of the 2010s, the cohesion of the European Union has been tested by several issues, including a debt crisis in some of the Eurozone countries , increasing migration from the Middle East , and the United Kingdom's withdrawal from the EU .  A referendum in the UK on its membership of the European Union was held on 23 June 2016, with % of participants voting to leave.  This is referred to in common parlance throughout Europe as Brexit , a portmanteau of "Britain" and "exit". The UK formally notified the European Council of its decision to leave on 29 March 2017 initiating the formal withdrawal procedure for leaving the EU, committing the UK to leave the EU on 29 March 2019. 
Under Article 49 of the Treaty on European Union any European country may apply for membership if it respects the democratic values of the EU and is committed to promoting them. A country wishing to join the EU submits a membership application to the Council of the European Union which asks the European Commission to assess the applicant’s ability to meet the conditions for membership (Copenhagen criteria) . If the Commission’s opinion is positive, the Council of the European Union must then agree upon a negotiating mandate. Negotiations are then formally opened.