The European Union sought to create ‘Europe without frontiers’. Elaborate.

The phrase means the removal of four distinct kinds of frontier, and the Union pursued each by a different instrument.

The economic frontier. The Treaty of Rome in 1957 established a common market resting on the four freedoms — movement of goods, services, capital and people. Progress stalled on non-tariff barriers until the Single European Act of 1986 set a deadline, and the single market was completed on 1 January 1993 through mutual recognition of standards rather than the impossible task of harmonising every rule.

The physical frontier. The Schengen Agreement of 1985 and its Convention of 1990, implemented from 1995 and folded into EU law by the Treaty of Amsterdam in 1997, abolished checks at internal borders. This is the most tangible sense in which frontiers disappeared: a traveller can cross most of the continent without producing a document.

The monetary frontier. Maastricht in 1992 created European Union citizenship and set the path to monetary union; the euro was introduced for accounting in 1999 and as notes and coin in 2002, and now circulates in twenty member states, removing exchange costs and currency risk within the area.

The political and social frontier. Direct elections to the European Parliament, the right to live and work in any member state, mutual recognition of qualifications, the Erasmus programme from 1987, and the enlargement of 2004 that brought ten mostly post-communist states in, ending the continent's Cold War division.

The reverses, which are now part of the story.

• Brexit in 2020 was the first withdrawal, and it restored a hard economic frontier where none had existed.
• The migration crisis of 2015 produced the reintroduction of internal border checks by several Schengen members, some of which have never been lifted.
• The external frontier hardened as the internal one softened: Frontex and the Union's border regime are the counterpart of free internal movement.
• Opt-outs on the euro and on Schengen mean the project was never uniform.

Assessment. Within its borders the Union achieved something historically unprecedented among sovereign states. It did so by moving the frontier outward rather than abolishing it, and the politics of that external frontier is now the Union's hardest problem.