This article is from the European Union FAQ, by Roland Siebelink & Bart Schelfhout with numerous contributions by others.
The Western European Union was founded in 1954 as a defence alliance between
the UK, France, the Netherlands, Belgium, Luxembourg, Germany and Italy,
after the rejection of the 1952 European Defence Community Treaty in 1954.
It was more or less dormant until the beginning of the 1990s, when it was
revived as a sort of common intermediary solution between an organisation of
the European NATO members and Defence aspirations of the European Union. It
was specifically mentioned as such in the Maastricht Treaty. Spain, Portugal
and Greece have joined the WEU since.
There are three categories of countries linked to the WEU without being full
members (with thanks to Richard Corbett[3] for explaining the difference:
Observer status EU Member states that are either not a member of NATO
(Austria, Finland, the Republic of Ireland and Sweden)
or that voluntarily refrain from full WEU membership
(Denmark);
Associate member status
Associate members are states that are European member
states of NATO but not of the EU (Norway and Turkey
[Is Iceland also in this category? RS])
Associate partner status Associate partners are European states that are
members of the NATO Partnership for Peace Initiative without being
full-blown members of either NATO or the EU: Bulgaria, Rumania, Hungary,
Czechia, Slovakia, Poland, Estonia, Lithuania, Latvia and Slovenia
 
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