The Alliance remains open to new members, according to article 10 of the Washington Treaty. Any European nation in a position to further the principles of the north Atlantic Treaty and contribute to the security in the Euro-Atlantic area, can join the alliance if invited to do so by the member countries. Since its formation, NATO has grown from 12 members, in 1949, to the current 26 members.

Following NATO’s establishment, there have been five separate occasions (1952, 1955, 1982, 1999 and 2004) on which new members were welcomed into the alliance. The single largest enlargement process took place in 2004 when seven new countries formally joined the alliance: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia. Prior to this, in 1999, the Czech Republic, Hungary and Poland became the first former members of the WP to join NATO.

In September 2008 Albania and Croatia were invited to begin accession talks with NATO. Allied leaders have also agreed to invite The Former Yugoslavia Republic of Macedonia, once issues with its name have been resolved. Bosnia and Herzegovina, Georgia, Montenegro and Ukraine have all started the process to join NATO in the future.