After its formation in 1949 with twelve founding members , NATO grew by including Greece and Turkey in 1952 and West Germany in 1955 , and then later Spain in 1982 . After the Cold War ended , and Germany reunited in 1990 , there was a debate in NATO about continued expansion eastward . In 1999 , Poland , Hungary , and the Czech Republic joined the organization , amid much debate within the organization and Russian opposition . Another expansion came with the accession of seven Central and Eastern European countries : Bulgaria , Estonia , Latvia , Lithuania , Romania , Slovakia , and Slovenia . These nations were first invited to start talks of membership during the 2002 Prague summit , and joined NATO shortly before the 2004 Istanbul summit . Albania and Croatia joined on 1 April 2009 , prior to the 2009 Strasbourg – Kehl summit . The most recent member state to be added to NATO is Montenegro on 5 June 2017
2002 Prague summit
2002 Prague summit
Enlargement of NATO