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, the 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 NATO 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 on 5 June 2017, and North Macedonia on 27 March 2020.
Russia invaded Ukraine in 2022 after Russia's president, Vladimir Putin, falsely claimed that NATO military infrastructure was being built up inside Ukraine and that Ukraine's potential future membership was a threat. Russia's invasion prompted Finland and Sweden to apply for NATO membership in May 2022.[1] Finland joined on 4 April 2023, and Sweden joined on 7 March 2024.[2][3][4] Ukraine applied for NATO membership in September 2022 after Russia proclaimed the annexation of its territory.[1] Two other states have formally informed NATO of their membership aspirations: Bosnia and Herzegovina and Georgia.[5] Kosovo also aspires to join NATO.[6] Joining the alliance is a debate topic in several other European countries outside the alliance, including Austria, Cyprus, Ireland, Malta, Moldova, and Serbia.[7]
Twelve countries were part of NATO at the time of its founding: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom, and the United States. The start of the Cold War between 1947 and 1953 saw an ideological and economic divide between the capitalist states of Western Europe backed by United States with its Marshall Plan and the Truman Doctrine, and the communist states of Eastern Europe, backed by the Soviet Union. As such, opposition to Soviet-style communism became a defining characteristic of the organization and the anti-communist governments of Greece, which had just fought a civil war against a pro-communist army, and Turkey, whose newly-elected Democrat Party were staunchly pro-American, came under internal and external pressure to join the alliance, which both did in February 1952.[8][9]
The US, France, and the UK initially agreed to end its occupation of Germany in May 1952 under the Bonn–Paris conventions on the condition that the new Federal Republic of Germany, commonly called West Germany, would join NATO, because of concerns about allowing a non-aligned West Germany to rearm. The allies also dismissed Soviet proposals of a neutral-but-united Germany as insincere.[10] France, however, delayed the start of the process, in part on the condition that a referendum be held in Saar on its future status, and a revised treaty was signed on 23 October 1954, allowing the North Atlantic Council to formally invite West Germany. Ratification of its membership was completed in May 1955.[11] That month the Soviet Union established its own collective defense alliance, commonly called the Warsaw Pact, in part as a response to West German membership in NATO.[12] In 1974, Greece suspended its NATO membership over the Turkish invasion of Cyprus, but rejoined in 1980 with Turkey's cooperation.[13]
Relations between NATO members and Spain under dictator Francisco Franco were strained for many years, in large part because Franco had cooperated with the Axis powers during World War II.[14] Though staunchly anti-communist, Franco reportedly feared in 1955 that a Spanish application for NATO membership might be vetoed by its members at the time.[15] Franco however did sign regular defense agreements with individual members, including the 1953 Pact of Madrid with the United States, which allowed use of its air and naval bases in Spain.[16][17] Following Franco's death in 1975, Spain began a transition to democracy, and came under international pressure to normalize relations with other western democracies. Prime Minister Adolfo Suárez, first elected in 1976, proceeded carefully on relations with NATO because of divisions in his coalition over the US's use of Spanish bases. In February 1981, following a failed coup attempt, Leopoldo Calvo-Sotelo became Prime Minister and campaigned strongly for NATO membership, in part to improve civilian control over the military, and Spain's NATO membership was approved in May 1982.[17][18] A Spanish referendum in 1986 confirmed popular support for remaining in NATO.[19]
During the mid-1980s the strength and cohesion of the Warsaw Pact, which had served as the main institution rivaling NATO, began to deteriorate. By 1989 the Soviet Union was unable to stem the democratic and nationalist movements which were rapidly gaining ground. Poland held multiparty elections in June 1989 that ousted the Soviet allied Polish Workers' Party and the peaceful opening of the Berlin Wall that November symbolized the end of the Warsaw Pact as a way of enforcing Soviet control. The fall of the Berlin Wall is recognized to be the end of the Cold War and ushered in a new period for Europe and NATO enlargement.[20]
Negotiations to reunite East and West Germany took place throughout 1990, resulting in the signing of the Two Plus Four Treaty in September 1990 and East Germany officially joining the Federal Republic of Germany on 3 October 1990. To secure Soviet approval of a united Germany remaining in NATO, the treaty prohibited foreign troops and nuclear weapons from being stationed in the former East Germany,[21] though an addendum signed by all parties specified that foreign NATO troops could be deployed east of the Cold War line after the Soviet departure at the discretion of the government of a united Germany.[22][23] There is no mention of NATO expansion into any other country in the September–October 1990 agreements on German reunification.[24] Whether or not representatives from NATO member states informally committed to not enlarge NATO into other parts of Eastern Europe during these and contemporary negotiations with Soviet counterparts has long been a matter of dispute among historians and international relations scholars.[25][26]
With several countries threatening to withdraw from the Warsaw Pact, the Soviet military relinquished control of the organization in March 1991, allowing it to be formally dissolved that July.[27][28]The so-called "parade of sovereignties" declared by republics in the Baltic and Caucasus regions of the Soviet Union and their War of Laws with the government in Moscow further fractured its cohesion. Following the failure of the New Union Treaty, the leadership of the remaining constituent republics of the Soviet Union, starting with Ukraine in August 1991, declared their independence and initiated the dissolution of the Soviet Union, which was completed in December of that year. Russia, led by President Boris Yeltsin, became the most prominent of the independent states.[29] The Westernization trend of many former Soviet allied states led them to privatize their economies and formalize their relationships with NATO countries, the first step for many towards European integration and possible NATO membership.[30][31]
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By August 1993, Polish President Lech Wałęsa was actively campaigning for his country to join NATO, at which time Yeltsin reportedly told him that Russia did not perceive its membership in NATO as a threat to his country. Yeltsin however retracted this informal declaration the following month,[32] writing that expansion "would violate the spirit of the treaty on the final settlement" which "precludes the option of expanding the NATO zone into the East."[33][34] During one of James Baker's 1990 talks with Soviet leader Mikhail Gorbachev, Baker did suggest that the German reunification negotiations could have resulted in an agreement whereby "there would be no extension of NATO's jurisdiction for forces of NATO one inch to the east,"[35] and historians like Mark Kramer have interpreted it as applying, at least in certain Soviet representatives' understanding, to all of Eastern Europe.[36][37][26] Gorbachev later stated that NATO expansion was "not discussed at all" in 1990,[38] but, like Yeltsin, described the expansion of NATO past East Germany as "a violation of the spirit of the statements and assurances made to us in 1990."[24][34][39]
This view, that informal assurances were given by diplomats from NATO members to the Soviet Union in 1990, is common in countries like Russia,[26][21] and, according to political scientist Marc Trachtenberg, available evidence suggests that allegations made since then by Russian leadership about the existence of such assurances "were by no means baseless."[25][40] Yeltsin was succeeded in 2000 by Vladimir Putin, who further promoted the idea that guarantees about enlargement were made in 1990, including during a 2007 speech in Munich.[41][39] This impression was later used by him as part of his justification for Russia's 2014 actions in Ukraine and the Russian invasion of Ukraine in 2022.[42][24]
In February 1991, Poland, Hungary, and Czechoslovakia formed the Visegrád Group to push for European integration under the European Union and NATO, as well as to conduct military reforms in line with NATO standards. Internal NATO reaction to these former Warsaw Pact countries was initially negative, but by the 1991 Rome summit in November, members agreed to a series of goals that could lead to accession, such as market and democratic liberalization, and that NATO should be a partner in these efforts. Debate within the American government as to whether enlargement of NATO was feasible or desirable began during the George H.W. Bush administration.[43] By mid-1992, a consensus emerged within the administration that NATO enlargement was a wise realpolitik measure to strengthen Euro-American hegemony.[43][44] In the absence of NATO enlargement, Bush administration officials worried that the European Union might fill the security vacuum in Central Europe, and thus challenge American post-Cold War influence.[43] There was further debate during the Presidency of Bill Clinton between a rapid offer of full membership to several select countries versus a slower, more limited membership to a wide range of states over a longer time span. Victory by the Republican Party, who advocated for aggressive expansion, in the 1994 US congressional election helped sway US policy in favor of wider full-membership enlargement, which the US ultimately pursued in the following years.[45] In 1996, Clinton called for former Warsaw Pact countries and post-Soviet republics to join NATO, and made NATO enlargement a part of his foreign policy.[46]
That year, Russian leaders like Foreign Minister Andrei Kozyrev indicated their country's opposition to NATO enlargement.[47] While Russian President Boris Yeltsin did sign an agreement with NATO in May 1997 that included text referring to new membership, he clearly described NATO expansion as "unacceptable" and a threat to Russian security in his December 1997 National Security Blueprint.[48] Russian military actions, including the First Chechen War, were among the factors driving Central and Eastern European countries, particularly those with memories of similar Soviet offensives, to push for NATO application and ensure their long-term security.[49][50] Political parties reluctant to move on NATO membership were voted out of office, including the Bulgarian Socialist Party in 1997 and Slovak HZDS in 1998.[51] Hungary's interest in joining was confirmed by a November 1997 referendum that returned 85.3% in favor of membership.[52] During this period, wider forums for regional cooperation between NATO and its eastern neighbors were set up, including the North Atlantic Cooperation Council (later the Euro-Atlantic Partnership Council) and the Partnership for Peace.[53]
While the other Visegrád members were invited to join NATO at its 1997 Madrid summit, Slovakia was excluded based on what several members considered undemocratic actions by nationalist Prime Minister Vladimír Mečiar.[54] Romania and Slovenia were both considered for invitation in 1997, and each had the backing of a prominent NATO member, France and Italy respectively, but support for this enlargement was not unanimous between members, nor within individual governments, including in the US Congress.[55] In an open letter to US President Bill Clinton, more than forty foreign policy experts including Bill Bradley, Sam Nunn, Gary Hart, Paul Nitze, and Robert McNamara expressed their concerns about NATO expansion as both expensive and unnecessary given the lack of an external threat from Russia at that time.[56] Hungary, Poland, and the Czech Republic officially joined NATO in March 1999.[57]
At the 1999 Washington summit NATO issued new guidelines for membership with individualized "Membership Action Plans" for Albania, Bulgaria, Estonia, Latvia, Lithuania, North Macedonia, Romania, Slovakia, and Slovenia in order to standardize the process for new members.[58] 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.[59] Slovenia had held a referendum on NATO the previous year, with 66% approving of membership.[60]
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.[61][59] Russian troops had been stationed in Baltic states as late as 1995,[62] but the goals of European integration and NATO membership were very attractive for the Baltic states.[63] Rapid investments in their own armed forces showed a seriousness in their desire for membership, and participation in NATO-led post-9/11 operations, particularly by Estonia in Afghanistan, won the three countries key support from individuals like US Senator John McCain, French President Jacques Chirac, and German Chancellor Gerhard Schröder.[62] A 2006 study in the journal Security Studies argued that the NATO enlargements in 1999 and 2004 contributed to democratic consolidation in Central and Eastern Europe.[64]