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 for the invitation of "other European States" only, and by subsequent agreements. Countries wishing to join must meet certain requirements and complete a multi-step process involving political dialog and military integration. The accession process is overseen by the North Atlantic Council, NATO's governing body. NATO formed in 1949 with twelve founding members, and has added new members eight times, with the first additions being Greece and Turkey in 1952. In May 1955, West Germany joined NATO, which was one of the conditions agreed to as part of the end of the country's occupation by France, the United Kingdom, and the United States. This prompted the Soviet Union to form their own collective security alliance, commonly called the Warsaw Pact, later that month. Following the end of the Franco regime, newly-democratic Spain chose to join NATO in 1982.
In 1990, the Soviet Union and NATO reached an agreement that a reunified Germany would join NATO under West Germany's pre-existing membership, although restrictions were agreed to on the deployment of NATO troops on former East German territory. The dissolution of the Soviet Union in 1991 led a number of former Warsaw Pact and post-Soviet states to initiate discussions about joining NATO. Poland, Hungary, and the Czech Republic became NATO members in 1999, amid much debate within NATO itself and Russian opposition. NATO then formalized the process of joining the organization with "Membership Action Plans", which aided the accession of seven Central and Eastern European countries shortly before the 2004 Istanbul summit: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, and Slovenia. Two countries on the Adriatic Sea—Albania and Croatia—joined on 1 April 2009 before the 2009 Strasbourg–Kehl summit. The most recent member states to join to NATO were Montenegro on 5 June 2017 and North Macedonia on 27 March 2020.
As of 2022[update], five additional states have formally informed NATO of their membership aspirations: Bosnia and Herzegovina, Finland, Georgia, Sweden, and Ukraine. Joining the alliance is a debate topic in several other European countries outside the alliance, including Ireland, Moldova, and Serbia.