NATO is a military alliance of thirty-two European and 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 dialogue and military integration. The accession process is overseen by the North Atlantic Council, NATO's governing body. NATO was formed in 1949 with twelve founding members and has added new members ten times. The first additions were 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, prompting the Soviet Union to form its 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, negotiators reached an agreement that a reunified Germany would be in NATO under West Germany's existing membership. Following the dissolution of the Soviet Union in 1991, many former Warsaw Pact and post-Soviet states sought to join NATO. Poland, Hungary, and the Czech Republic became members in 1999, amid much debate within NATO itself. NATO then formalized the process of joining the organization with "Membership Action Plans", which aided the accession of seven Central and Eastern Europe 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 next member states to join NATO were Montenegro in June 2017, and North Macedonia in March 2020.
