Enlargement of NATO

NATO is a military alliance of thirty-two European and North American countries that work together for their shared safety. It began in 1949 with twelve founding members and has grown over time. The process for a country to join NATO is careful and involves meeting certain rules and having talks with existing members.

In the past, countries like Greece, Turkey, and West Germany joined in the 1950s. After the end of the Soviet Union in 1991, many countries from Central and Eastern Europe wanted to become part of NATO. Poland, Hungary, and the Czech Republic joined in 1999, and several others followed in the years after.

In 2022, Russia invaded Ukraine, which led Finland and Sweden to decide to join NATO. Finland became a member in 2023, and Sweden joined in 2024. Ukraine also applied to join after Russia took control of part of its land. Today, many other countries in Europe are thinking about whether they should join NATO, and discussions continue about what that might mean for the future of the alliance.

Past enlargements

See also: Member states of NATO and Controversy regarding NATO's eastward expansion

Cold War

Twelve countries were part of NATO when it began: 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 a split between the capitalist states of Western Europe backed by the United States and the communist states of Eastern Europe, backed by the Soviet Union. Opposition to Soviet-style communism became a key part of NATO, 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 strongly pro-American, joined the alliance in February 1952.

The US, France, and the UK agreed to end their occupation of Germany in May 1952 on the condition that West Germany would join NATO. France delayed the process, but a revised treaty was signed in October 1954, allowing West Germany to join. Ratification was completed in May 1955. That month, the Soviet Union created its own defense alliance, called the Warsaw Pact, partly in response to West Germany joining NATO. In 1966, French president Charles de Gaulle announced the withdrawal of French forces from NATO's military structure. In 1974, Greece paused its NATO membership over the Turkish invasion of Cyprus, but rejoined in 1980 with Turkey's help.

Relations between NATO members and Spain under dictator Francisco Franco were difficult for many years because Franco had worked with the Axis powers during World War II. Though anti-communist, Franco feared that Spain might be blocked from joining NATO. He signed defense agreements with individual members, including the 1953 Pact of Madrid with the United States. After Franco's death in 1975, Spain began a transition to democracy and came under pressure to normalize relations with other western democracies. In February 1981, after a failed coup attempt, Prime Minister Leopoldo Calvo-Sotelo strongly supported NATO membership. Spain's NATO membership was approved in May 1982. A Spanish referendum in 1986 confirmed support for staying in NATO.

During the mid-1980s, the strength of the Warsaw Pact, which rivaled NATO, began to weaken. By 1989, the Soviet Union could not stop democratic and nationalist movements gaining support. Poland held multiparty elections in June 1989 that ended the rule of the Soviet allied Polish Workers' Party, and the peaceful opening of the Berlin Wall that November symbolized the end of the Warsaw Pact. The fall of the Berlin Wall is seen as the end of the Cold War and started a new period for Europe and NATO enlargement.

German reunification

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 stopped foreign troops and nuclear weapons from being stationed in the former East Germany, though an addendum allowed foreign NATO troops to be deployed east of the Cold War line after the Soviet departure. There is no mention of any other country joining NATO in the September–October 1990 agreements on German reunification.

With several countries threatening to leave the Warsaw Pact, the Soviet military gave up control of the organization in March 1991, allowing it to be formally dissolved that July. Republics in the Baltic and Caucasus regions of the Soviet Union declared their independence and started 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. Many former Soviet allied states moved toward European integration and possible NATO membership.

Visegrád Group

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. NATO reaction to these former Warsaw Pact countries was initially negative, but by the 1991 Rome summit in November, members agreed to goals that could lead to accession, such as market and democratic liberalization, and that NATO should support these efforts. Debate within the US government began during the George H.W. Bush administration. By mid-1992, a consensus emerged within the administration that NATO enlargement was wise to strengthen Euro-American influence. 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. The Partnership for Peace provided a structure for NATO to expand military and political cooperation with former Warsaw Pact member states, including Russia, while postponing immediate decisions on formal membership for any state. Victory by the Republican Party 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. In 1996, Clinton called for former Warsaw Pact countries and post-Soviet republics to join NATO, and made NATO enlargement part of his foreign policy.

By August 1993, Polish President Lech Wałęsa was actively campaigning for his country to join NATO. In 1996, Russian Foreign Minister Andrei Kozyrev indicated his country's opposition to NATO enlargement. Russian President Boris Yeltsin signed the NATO-Russia Founding Act in May 1997, which affirms that "NATO and Russia do not consider each other as adversaries". The agreement refers to "new members". Though Yeltsin called NATO enlargement a mistake, he said "the negative consequences of NATO's enlargement will be reduced to the minimum through the NATO–Russia deal". Russia's December 1997 National Security Blueprint described NATO enlargement eastwards as "unacceptable" and a threat to Russian security. Russian military actions were among the factors driving Central and Eastern European countries to push for NATO application and ensure their long-term security. Hungary's interest in joining was confirmed by a November 1997 referendum that returned 85.3% in favor of membership. 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.

While the other Visegrád members were invited to join NATO at its 1997 Madrid summit, Slovakia was excluded based on undemocratic actions by nationalist Prime Minister Vladimír Mečiar. Romania and Slovenia were both considered for invitation in 1997, but support for this enlargement was not unanimous between members, nor within individual governments, including in the US Congress. Hungary, Poland, and the Czech Republic officially joined NATO in March 1999.

Vilnius Group

In 1997, U.S. policy towards the Baltic States shifted towards active support for NATO membership, along with greater military participation and cooperation in Partnership for Peace exercises with the three states.

By 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. 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. Slovenia had held a referendum on NATO the previous year, with 66% approving of membership.

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. Russian troops had been stationed in Baltic states as late as 1995, and Russia had proposed alternative security, economic, and diplomatic arrangements in the late 90s, but the goals of European integration and NATO membership remained attractive for the Baltic states. 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. 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.

Adriatic Charter

Croatia also started a Membership Action Plan at the 2002 summit, but was not included in the 2004 enlargement. In May 2003, it joined with Albania and Macedonia to form the Adriatic Charter to support each other in their pursuit of membership. Croatia's prospect of membership sparked a national debate on whether a referendum on NATO membership needed to be held before joining the organization. Croatian Prime Minister Ivo Sanader ultimately agreed in January 2008, as part of forming a coalition government with the HSS and HSLS parties, not to officially propose one. Albania and Croatia were invited to join NATO at the 2008 Bucharest summit that April, though Slovenia threatened to hold up Croatian membership over their border dispute in the Bay of Piran. Slovenia did ratify Croatia's accession protocol in February 2009, before Croatia and Albania both officially joined NATO just before the 2009 Strasbourg–Kehl summit, with little opposition from Russia.

Montenegro declared independence on 3 June 2006; the new country subsequently joined the Partnership for Peace program at the 2006 Riga summit and then applied for a Membership Action Plan on 5 November 2008, which was granted in December 2009. Montenegro also began full membership with the Adriatic Charter of NATO aspirants in May 2009. NATO formally invited Montenegro to join the alliance on 2 December 2015, with negotiations concluding in May 2016; Montenegro joined NATO on 5 June 2017.

Macedonia joined the Partnership for Peace in 1995, and commenced its Membership Action Plan in 1999, at the same time as Albania. At the 2008 Bucharest summit, Greece blocked a proposed invitation because it believed that the official name it was using at the time, "Republic of Macedonia," implied territorial aspirations toward its own region of Greek Macedonia. NATO nations agreed that the country would receive an invitation upon resolution of the Macedonia naming dispute. Macedonia sued Greece at the International Court of Justice (ICJ) over Greece's veto of Macedonia's NATO membership. Macedonia was part of the Vilnius Group, and had formed the Adriatic Charter with Croatia and Albania in 2003 to better coordinate NATO accession.

In June 2017, Macedonian Prime Minister Zoran Zaev signaled he would consider alternative names for the country in order to strike a compromise with Greece, settle the naming dispute and lift Greek objections to Macedonia joining the alliance. The naming dispute was resolved with the Prespa Agreement in June 2018 under which the country adopted the name North Macedonia, which was supported by a referendum in September 2018. NATO invited North Macedonia to begin membership talks on 11 July 2018; formal accession talks began on 18 October 2018. NATO's members signed North Macedonia's accession protocol on 6 February 2019. Most countries ratified the accession treaty in 2019, with Spain ratifying its accession protocol in March 2020. The Sobranie also ratified the treaty unanimously on 11 February 2020, before North Macedonia became a NATO member state on 27 March 2020.

Finland and Sweden

Further information: Finland–NATO relations and Sweden–NATO relations

In 1949, Sweden chose not to join NATO and declared a security policy aiming for non-alignment in peace and neutrality in war. This position was maintained without much discussion during the Cold War. During this time, Finland's relationship with NATO and the Soviet Union followed the Paasikivi–Kekkonen doctrine, whereby the country joined neither the Western nor Eastern blocs. Following the dissolution of the Soviet Union in 1991, both countries joined NATO's Partnership for Peace in 1994 and provided peacekeeping forces to various NATO missions, including Kosovo (KFOR) and Afghanistan (ISAF) in the early 2000s.

Russia's annexation of Crimea from Ukraine in 2014 and subsequent full-scale invasion of Ukraine in 2022 led both countries to revisit their security and defence policies. Opinion polls in both countries shortly after the 2022 invasion for the first time showed clear majorities supported joining NATO. The major political parties in both also re-evaluated their positions on NATO membership, with many moving to either support membership or drop long-standing opposition. On 15 May 2022, Finland's Prime Minister Marin and President Niinistö together announced that Finland would apply for NATO membership, while Swedish Prime Minister Magdalena Andersson announced that Sweden would do the same the following day. Formal applications for membership were jointly submitted by both countries on 18 May 2022.

Turkey, however, opposed the start of accession negotiations for Finland and Sweden over multiple issues, most notably their claim that Finland and Sweden supported the Kurdish groups like the PKK, PYD, and YPG, which Turkey views as terrorists, and the followers of Fethullah Gülen, whom Turkey accused of orchestrating the unsuccessful 2016 Turkish coup d'état attempt. Accession negotiations only began following the signing of a tripartite memorandum between Finland, Sweden, and Turkey during the NATO summit in Madrid in June 2022. That memorandum allowed for formal invitations and for the ratification process to begin the following month, and by that October, only Hungary and Turkey had not approved the pair's applications.

Negotiations between the countries continued, and in February 2023, Turkish President Recep Tayyip Erdoğan announced that he now had a positive view of Finland's membership, but still a negative view of Sweden's. After the Hungarian and Turkish parliaments both approved Finland's application in March, Finland became a member of the alliance on 4 April 2023, the 74th anniversary of the North Atlantic Treaty being signed. Turkey and Sweden finally came to an understanding in early 2024, allowing Sweden to become the 32nd member of the alliance on 7 March 2024.

According to the Russian defence minister Sergei Shoigu, Finland's accession to NATO significantly increased the risk of a wider conflict in Europe. The move doubled the length of the border that the North Atlantic Treaty Organization shares with Russia. Russian President Vladimir Putin, however, has consistently dismissed Finland's and Sweden's accession to NATO, stating it poses "no threat to Russia".

Summary table and map

Criteria and process

Article 10 and the Open Door Policy

Main article: NATO open door policy

The North Atlantic Treaty is the foundation of NATO, and any changes, including new members, need approval from all current members. Article 10 of the treaty explains how countries can join NATO. It says that any European country that supports the treaty’s principles and helps keep the North Atlantic area safe can be invited to join. But, only European countries can join, and all current members must agree.

There are some rules. For example, Greece did not want Republic of Macedonia to join because of a name dispute. Similarly, Turkey has concerns about Republic of Cyprus joining until a dispute is solved.

Membership Action Plan

In 1999, NATO created the Membership Action Plan (MAP) to help countries work towards joining. Countries in MAP must show progress in five areas each year: solving disputes peacefully, respecting laws and human rights, having democratic control of the military, being able to help NATO’s missions, and keeping information safe. NATO gives feedback and advice. Once a country meets the requirements, NATO can invite it to start joining.

In 2002, seven countries were invited to join through MAP: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, and Slovenia. They all joined in 2004. Other countries like Albania, Croatia, Montenegro, and North Macedonia also joined later. As of 2026^[update], Bosnia and Herzegovina is still in a MAP.

Intensified dialogue

In 2005, NATO started “Intensified Dialogue” for countries interested in joining, like Ukraine. This means talking about many topics related to NATO membership. Later, Georgia also joined this dialogue. Other countries like Montenegro, Bosnia and Herzegovina, and Serbia were offered this chance too.

Aspiring members

As of January 2025, three states have shown they want to join NATO. Bosnia and Herzegovina is the only country with a plan to start the joining process, along with Georgia. Both were called "aspirant countries" by NATO leaders in December 2011.

Bosnia and Herzegovina

Bosnia and Herzegovina has wanted to join NATO for many years. After wars and peace agreements in the 1990s, NATO helped keep peace there. Bosnia joined programs to work closely with NATO in 2006 and started formal talks about joining in 2008. The country faces challenges because of its divided government, but many people there support joining NATO.

Georgia

Georgia also wants to join NATO. After political changes in 2003, Georgia moved to become closer to NATO. However, Georgia’s neighbor, Russia, has opposed this. Despite tensions, Georgia continues to work on reforms and partnerships with NATO.

Ukraine

Ukraine’s relationship with NATO has changed over time. Ukraine started working with NATO in the early 2000s. After political changes and conflict with Russia, Ukraine renewed its efforts to join NATO. In 2022, after Russia invaded Ukraine, many people in Ukraine supported joining NATO more than ever before.

Membership debates

Further information: Foreign relations of NATO

After the end of the Cold War, several countries that were neutral or had recently become independent started to build closer ties with Western groups. Some of these countries asked to join NATO. The 2022 Russian attack on Ukraine brought new discussions about NATO membership in many countries.

Austria, Ireland, Switzerland, and Malta have kept their neutral status from the Cold War. All are now part of the Partnership for Peace, and all except Switzerland are also members of the European Union. Switzerland’s government looked at ways to work more closely with NATO in May 2022. A survey that month showed 33% of Swiss people supported joining NATO, while 56% wanted closer ties. Cyprus is part of the European Union but is not a full NATO member and does not take part in the Partnership for Peace. Any agreement for Cyprus to join NATO might face challenges because of disagreements with Turkey.

Russia, Armenia, Belarus, and Kazakhstan are all members of the Collective Security Treaty Organization, a military group formed after the Soviet Union ended. Azerbaijan used to be a member but chose to stay neutral in 1999. In 2000, the leader of Russia, Vladimir Putin, talked about possibly joining NATO, but these ideas did not move forward. From the early 2000s, Putin began to speak against NATO.

Armenia

Further information: Armenia–NATO relations

Armenia became independent from the Soviet Union in 1991. Since then, Armenia has worked to build stronger ties with NATO countries. Armenia joined the Partnership for Peace program in 1994. Some political groups in Armenia have suggested leaving a different military group and either joining NATO fully or becoming a close partner with NATO.

After fights between Armenia and Azerbaijan in September 2022, Armenia felt it did not get enough help from its current military group. This led many people in Armenia to question whether staying in that group was a good idea, with some calling for closer ties with NATO instead. A visit from a senior U.S. leader to Armenia in September 2022 was seen as a sign of support for this change.

In November 2022, protests in Armenia called for leaving the current military group and building closer ties with the United States and the West. Some U.S. leaders have also supported the idea of Armenia becoming a closer partner with the U.S. In September 2023, Armenia’s leader said depending only on one country for security was a mistake. In February 2024, Armenia’s leader said the country had stopped taking part in its current military group and was thinking about leaving. In June 2024, Armenia said it would leave the group at some point in the future. In May 2024, Armenia’s government asked to start the process of joining the European Union, though they do not plan to join NATO.

Austria

Further information: Austria–NATO relations

Austria was controlled by Allied powers after World War II. During talks to end this control, the Soviet Union wanted Austria to stay neutral like Switzerland. Austria became independent in 1955 and soon after promised to stay neutral, which means it cannot join a military group, host foreign military bases, or go to war.

Austria joined the European Union in 1995 along with Sweden and Finland, all of whom had been neutral during the Cold War. Austria also joined NATO’s Partnership for Peace in 1995. Austria’s army takes part in United Nations peacekeeping missions in places like Kosovo, Lebanon, and Bosnia and Herzegovina.

After the 2022 Russian attack on Ukraine, some in Austria talked about changing the country’s neutral status, but the current government and many people still oppose joining NATO. A survey in March 2023 showed only 21% of Austrians supported joining NATO, while 61% were against it.

Cyprus

Further information: Cyprus–NATO relations

Before becoming independent in 1960, Cyprus was a British colony and part of NATO through the United Kingdom. After independence, Cyprus chose to stay neutral. In 1974, Turkey invaded Cyprus, which makes it hard for Cyprus to join NATO because Turkey, a NATO member, opposes it.

In 2011, Cyprus’s parliament voted to join NATO’s Partnership for Peace, but the president at the time refused, saying it could hurt efforts to solve the dispute with Turkey. In May 2022, Cyprus said it would not apply to join NATO. However, after an election in 2023, the new president changed this position and plans to build closer ties with NATO, including joining some NATO-linked groups, modernizing its army, and working more with the U.S. The president says the U.S. has responded positively to these plans.

Ireland

Further information: Ireland–NATO relations

Ireland stayed neutral during World War II and kept this policy during the Cold War. Ireland’s law requires government, parliament, and United Nations approval before sending troops abroad, known as the “triple lock.” Ireland has sent small groups of troops to support NATO missions in Afghanistan and Kosovo.

Ireland joined NATO’s Partnership for Peace and the Euro-Atlantic Partnership Council in 1999. This helps Ireland work with NATO members on training and meeting international standards for peacekeeping missions. In February 2024, Ireland renewed its agreement with NATO, adding cybersecurity and underseas cable security as new areas of cooperation.

Some politicians in Ireland support joining NATO, but no major party currently does. Public opinion has shifted over time; after the 2022 Russian attack on Ukraine, more people supported joining NATO, but by June 2023, opinions returned to previous levels. Former NATO leader Anders Fogh Rasmussen said Ireland’s door to NATO is always open.

Kosovo

Main article: Kosovo–NATO relations

Kosovo, which declared independence from Serbia in 2008, wants to join NATO. Kosovo applied to join the Partnership for Peace in 2012 and aims to become a NATO member by 2022. However, some NATO members do not recognize Kosovo, which could make joining harder. Kosovo is not a member of the United Nations, which is usually needed for NATO membership.

In February 2022, during the Russian attack on Ukraine, Kosovo asked for a permanent U.S. military base and faster NATO membership, saying there is an urgent need for peace and stability in the Western Balkans. In March 2022, Kosovo’s parliament passed a resolution asking the government to take steps to join NATO, the European Union, the Council of Europe, and other international groups.

Malta

Main article: Malta–NATO relations

When NATO was created in 1949, Malta, a British colony, was part of NATO through the United Kingdom. Malta became independent in 1964 but did not join NATO. From 1952 to 1965, NATO’s Mediterranean forces were based in Malta. The leader at the time wanted to join NATO but was discouraged by NATO’s deputy secretary general.

In 1971, Malta’s new leader chose neutrality, which was later written into the country’s constitution. Malta joined the Non-Aligned Movement in 1979 when Britain left its naval base there. In 1995, Malta joined NATO’s Partnership for Peace and the Euro-Atlantic Partnership Council, but left both when a different party took power in 1996. Malta rejoined in 2008 after joining the European Union in 2004. No political party in Malta supports joining NATO. A survey in February 2022 showed most Maltese people support neutrality, though some support closer military cooperation within the European Union.

Moldova

Further information: Moldova–NATO relations and Moldovan neutrality

Moldova became independent in 1991 after the Soviet Union ended. Its constitution does not allow joining a military alliance, but some leaders have talked about joining NATO as part of becoming more European. Moldova joined NATO’s Partnership for Peace in 1994 and started an Individual Partnership Action Plan in 2010. Moldova also helps with NATO’s peacekeeping in Kosovo.

After Russia’s 2014 takeover of Crimea, NATO warned Russia might try to take over Transnistria, a region in Moldova that wants to be separate. This could make it hard for Moldova to join NATO. Moldova’s current president has talked about joining a bigger alliance but did not name NATO specifically. A survey in 2018 showed only 22% of Moldovans would vote to join NATO if given the choice, while 32% would vote no.

Serbia

Further information: Serbia–NATO relations

Serbia’s history with NATO is complicated. NATO bombed Serbia in 1999 during the Kosovo War. After this, Serbia wanted better relations with NATO but joining remained unpopular. In 2007, Serbia’s parliament said the country would stay neutral until a vote could be held. Serbia joined NATO’s Partnership for Peace in 2006 and started an intensified dialog program in 2008.

Serbia’s current president says the country is not interested in joining NATO. A survey in March 2022 showed 82% of Serbians opposed joining NATO, while only 10% supported it. Serbia has close ties with Russia, especially on issues like Kosovo, and did not support sanctions against Russia after its invasion of Ukraine.

Other proposals

Some people have talked about letting countries outside of Europe join NATO. But the rules say only European countries can join, so changing this would need to update the rules.

People have suggested countries like Mexico, Colombia, Qatar, and Brazil join NATO. In 2013, a leader from Colombia talked about working more closely with NATO, but later said they were not asking to join. In 2018, Qatar asked to join, but NATO said no. In 2019, the leader of the United States supported Brazil joining NATO someday, but France said only European countries can join.

Others have suggested letting democratic countries from places like Asia and the Pacific join NATO. This idea includes countries like Australia, New Zealand, Japan, and South Korea. Some have also mentioned Singapore and Israel. There have also been ideas about Middle Eastern countries joining.

Sometimes, new countries could form inside an existing NATO member and want to join NATO too. For example, if Scotland became its own country, it said it would want to stay in NATO. Similarly, if Catalonia became independent from Spain, it said it would want to keep its place in NATO. However, some other countries worry this could encourage other areas to try to break away.