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An animation showing the year and location of counties as they joined the alliance.
Map of NATO countries' chronological membership

Enlargement of the North Atlantic Treaty Organization (NATO) is the process of including new member states in NATO. NATO is a military alliance of twenty-eight European and two North American countries that constitutes a system of collective defense. The process of joining the alliance is governed by Article 10 of the North Atlantic Treaty, which allows only for the invitation of "other European States", and by subsequent agreements. Countries wishing to join have to meet certain requirements and complete a multi-step process involving political dialogue and military integration. The accession process is overseen by the North Atlantic Council, NATO's governing body.

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 NATO, 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 states to be added to NATO are Montenegro on 5 June 2017 and North Macedonia on 27 March 2020.

As of 2020, NATO officially recognizes Bosnia and Herzegovina as an official candidate and two aspiring members: Georgia and Ukraine. Future expansion is currently a topic of debate in several countries outside the alliance, and countries like Sweden, Finland, and Serbia have open political debate on the topic of membership, while in countries like Ukraine, support and opposition to membership is tied to ethnic and nationalist ideologies. The incorporation of countries formerly part of the Eastern Bloc and the Soviet Union has been a cause of increased tension between NATO countries and Russia.