North Atlantic Treaty Organisation (NATO) - international alliance for collective defense created on 4 April 1949 through the signing of The North Atlantic Treaty in Washington. The document has been signed by the ministers of foreign affairs of twelve founding states (Belgium, Denmark, Holland, France, Iceland, Canada, Luxembourg, Norway, Portugal, USA, Italy and United Kingdom).
Article 5 - An armed attack against one or more members in Europe or North America shall be considered an attack against them all.
Article 10 - The parties may, by unanimous agreement, invite any other European state in a position to further the principles of the Treaty and to contribute to the security of the North Atlantic area to accede to this Treaty.
Enlargement Process:
1952 - Greece and Turkey
1955 - Federal Republic of Germany
1982 - Spain
1999 - Czech Republic, Hungary and Poland
2004 - Bulgaria, Estonia, Latvia, Lituania, Romania, Slovakia and Slovenia
2009 - Albania, Croatia
At the moment the Alliance has 28 members including states from Europe and North America.