Explanation

NATO (North Atlantic Treaty Organization) is an intergovernmental military alliance established in 1949, built on the principle of collective defense — an attack against one member is considered an attack against all. Its headquarters is in Brussels, Belgium, and it currently has 32 member countries as of 2024.

Austria is not a NATO member. It has maintained a policy of permanent neutrality since 1955, following the Austrian State Treaty, and has consistently stayed outside military alliances.

Bulgaria is a NATO member, having joined in 2004 as part of NATO’s major post-Cold War expansion into Eastern Europe, alongside six other countries including Romania, Slovakia, and the Baltic states.

Croatia is a NATO member, having joined on 1 April 2009 along with Albania, after depositing their instruments of accession.

Serbia is not a NATO member. It follows a military neutrality policy and has historical, cultural, and political ties with Russia, making NATO membership a contentious issue domestically.

Sweden is a NATO member, having joined as the 32nd member in March 2024. Sweden ended its longstanding policy of neutrality after applying for membership in 2022 alongside Finland, primarily in response to Russia’s invasion of Ukraine and evolving European security concerns.

North Macedonia is a NATO member, having joined as the 30th member in 2020 after resolving a long-standing name dispute with Greece — which had previously blocked its accession — through the Prespa Agreement of 2018.

Therefore, Bulgaria, Croatia, Sweden, and North Macedonia — four out of six — are NATO members.