At the 1999 Washington summit , where Hungary , Poland , and the Czech Republic officially joined , NATO also issued new guidelines for membership with individualized " Membership Action Plans " for Albania , Bulgaria , Estonia , Latvia , Lithuania , North Macedonia , Romania , Slovakia , and Slovenia . In May 2000 , these countries joined with Croatia to form the Vilnius Group in order to cooperate and lobby for common NATO membership , and by the 2002 Prague summit seven were invited for membership , which took place at the 2004 Istanbul summit . Russia was particularly upset with the addition of the three Baltic states , the first countries that were part of the Soviet Union to join NATO
1999 Washington summit
1999 Washington summit
Enlargement of NATO