Enlargement and Article 10

  • Last updated: 08 Mar. 2024 12:04

NATO’s door remains open to any European country in a position to undertake the commitments and obligations of membership, and contribute to security in the Euro-Atlantic area. Since 1949, NATO’s membership has increased from 12 to 32 countries through 10 rounds of enlargement. Sweden became the latest country to join the Alliance on 7 March 2024. Currently, three partner countries have declared their aspirations to NATO membership: Bosnia and Herzegovina, Georgia and Ukraine.

Raising of the flag of Sweden at the Accession Ceremony

Sweden became the latest country to join the Alliance on 7 March 2024.

  • NATO's "open door policy" is based on Article 10 of its founding treaty. Any decision to invite a country to join the Alliance is taken by the North Atlantic Council on the basis of consensus among all Allies. No third country has a say in such deliberations.
  • NATO's ongoing enlargement process poses no threat to any country. It is aimed at promoting stability and cooperation, at building a Europe whole and free, united in peace, democracy and common values.
  • At the 2008 Bucharest Summit, the Allies agreed that Georgia and Ukraine will become members of NATO in future.
  • Bosnia and Herzegovina was invited to join the Membership Action Plan (MAP) in April 2010.
  • Following Russia's full-scale invasion of Ukraine in February 2022, the Finnish and Swedish ambassadors to NATO simultaneously conveyed the intent of their respective countries to join the Alliance by submitting official letters of application to NATO on 18 May. They completed accession talks on 4 July, and their Accession Protocols were signed by Allies on 5 July. Thereafter, once NATO countries had ratified the Accession Protocols according to their national procedures, Finland and Sweden became full members of the Alliance on 4 April 2023 and 7 March 2024, respectively. 



Aspirant countries

Countries that have declared an interest in joining the Alliance are initially invited to engage in an intensified dialogue with NATO about their membership aspirations and related reforms.

Aspirant countries may then be invited to participate in the MAP to prepare for potential membership and demonstrate their ability to meet the obligations and commitments of possible future membership. Participation in the MAP does not guarantee membership.

Countries aspiring to join NATO have to demonstrate that they are in a position to further the principles of the 1949 Washington Treaty and contribute to security in the Euro-Atlantic area. They are also expected to meet certain political, economic and military criteria, which are laid out in the 1995 Study on NATO Enlargement.


1995 Study on Enlargement

In 1995, the Alliance published the results of a Study on NATO Enlargement that considered the merits of admitting new members and how they should be brought in. It concluded that the end of the Cold War provided a unique opportunity to build improved security in the entire Euro-Atlantic area and that NATO enlargement would contribute to enhanced stability and security for all. It would do so, the Study further concluded, by encouraging and supporting democratic reforms, including the establishment of civilian and democratic control over military forces; fostering patterns and habits of cooperation, consultation and consensus-building characteristic of relations among members of the Alliance; and promoting good-neighbourly relations.

It would increase transparency in defence planning and military budgets, thereby reinforcing confidence among states, and would reinforce the overall tendency toward closer integration and cooperation in Europe. The Study also concluded that enlargement would strengthen the Alliance's ability to contribute to European and international security and strengthen and broaden the transatlantic partnership.

According to the Study, countries seeking NATO membership would have to be able to demonstrate that they have fulfilled certain requirements. These include:

  • a functioning democratic political system based on a market economy;
  • the fair treatment of minority populations;
  • a commitment to the peaceful resolution of conflicts;
  • the ability and willingness to make a military contribution to NATO operations; and
  • a commitment to democratic civil-military relations and institutional structures.

Once admitted, new members would enjoy all the rights and assume all the obligations of membership. This would include acceptance at the time that they join of all the principles, policies and procedures previously adopted by Alliance members.


Accession process

Once the Allies have decided to invite a country to become a member of NATO, they officially invite the country to begin accession talks with the Alliance. This is the first step in the accession process on the way to formal membership. The major steps in the process are:

1. Accession talks with a NATO team

These talks take place at NATO Headquarters in Brussels and bring together teams of NATO experts and representatives of the individual invitees. Their aim is to obtain formal confirmation from the invitees of their willingness and ability to meet the political, legal and military obligations and commitments of NATO membership, as laid out in the Washington Treaty and in the Study on NATO Enlargement.

The talks take place in two sessions with each invitee. In the first session, political and defence or military issues are discussed, essentially providing the opportunity to establish that the preconditions for membership have been met. The second session is more technical and includes discussion of resources, security, and legal issues as well as the contribution of each new member country to NATO's common budget. This is determined on a proportional basis, according to the size of their economies in relation to those of other Alliance member countries.

Invitees are also required to implement measures to ensure the protection of NATO classified information, and prepare their security and intelligence services to work with the NATO Office of Security.

The end product of these discussions is a timetable to be submitted by each invitee for the completion of necessary reforms, which may continue even after these countries have become NATO members.

2. Invitees send letters of intent to NATO, along with timetables for completion of reforms

In the second step of the accession process, each invitee country provides confirmation of its acceptance of the obligations and commitments of membership in the form of a letter of intent from each foreign minister addressed to the NATO Secretary General. Together with this letter they also formally submit their individual reform timetables.

3. Accession protocols are signed by NATO countries

NATO then prepares Accession Protocols to the Washington Treaty for each invitee. These protocols are in effect amendments or additions to the Treaty, which once signed and ratified by Allies, become an integral part of the Treaty itself and permit the invited countries to become parties to the Treaty.

4. Accession protocols are ratified by NATO countries

The governments of NATO member states ratify the protocols, according to their national requirements and procedures. The ratification procedure varies from country to country. For example, the United States requires a two-thirds majority to pass the required legislation in the Senate. Elsewhere, for example in the United Kingdom, no formal parliamentary vote is required.

5. The Secretary General invites the potential new members to accede to the North Atlantic Treaty

Once all NATO member countries notify the Government of the United States of America, the depository of the Washington Treaty, of their acceptance of the protocols to the North Atlantic Treaty on the accession of the potential new members, the Secretary General invites the new countries to accede to the Treaty.

6. Invitees accede to the North Atlantic Treaty in accordance with their national procedures

7. Upon depositing their instruments of accession with the US State Department, invitees formally become NATO members


Evolution of NATO's "open door policy"

NATO's "open door policy" is based upon Article 10 of the Washington Treaty, which states that membership is open to any "European State in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area".

The enlargement of the Alliance is an ongoing and dynamic process. Since the Alliance was created in 1949, its membership has grown from the 12 founding members to today's 32 members through 10 rounds of enlargement in 1952, 1955, 1982, 1999, 2004, 2009, 2017, 2020, 2023 and 2024.

The first three rounds of enlargement – which brought in Greece and Türkiye (1952), West Germany (1955) and Spain (1982) – took place during the Cold War, when strategic considerations were at the forefront of decision-making.

The fall of the Berlin Wall in November 1989 signalled the end of the Cold War and was followed by the dissolution of the Warsaw Pact and the breakup of the Soviet Union. The reunification of Germany in October 1990 brought the territory of the former East Germany into the Alliance. The new democracies of Central and Eastern Europe were eager to guarantee their freedom by becoming integrated into Euro-Atlantic institutions.

NATO enlargement was the subject of lively debate in the early 1990s. Many political analysts were unsure of the benefits that enlargement would bring. Some were concerned about the possible impact on Alliance cohesion and solidarity, as well as on relations with other states, notably Russia. It is in this context that the Alliance carried out a Study on NATO Enlargement in 1995 (see above).

Post-Cold War enlargement

Based on the findings of the Study on Enlargement, the Alliance invited Czechia, Hungary and Poland to begin accession talks at the Alliance's Madrid Summit in 1997. These three countries became the first former members of the Warsaw Pact to join NATO in 1999.

At the 1999 Washington Summit, the Membership Action Plan was launched to help other aspirant countries prepare for possible membership.

Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia were invited to begin accession talks at the Alliance's Prague Summit in 2002 and joined NATO in 2004. All seven countries had participated in the MAP.

At the Bucharest Summit in April 2008, Allied leaders took a number of steps related to the future enlargement of the Alliance. Several decisions concerned countries in the Western Balkans (see Milestones below). The Allies see the closer integration of Western Balkan countries into Euro-Atlantic institutions as essential to ensuring long-term self-sustaining stability in this region, where NATO has been heavily engaged in peace-support operations since the mid-1990s. Albania and Croatia joined the Alliance in April 2009; Montenegro in June 2017; and the Republic of North Macedonia in March 2020.

Allied leaders also agreed at Bucharest that Georgia and Ukraine, which were already engaged in Intensified Dialogues with NATO, will one day become members. In December 2008, Allied foreign ministers decided to enhance opportunities for assisting the two countries in efforts to meet membership requirements by making use of the framework of the NATO-Ukraine Commission (which has since become the NATO-Ukraine Council) and the NATO-Georgia Commission – without prejudice to further decisions which may be taken about their applications to join the MAP. (For further information on NATO's cooperation with Georgia and Ukraine see pages on Relations with Georgia and Relations with Ukraine.)

In May 2022, Finland and Sweden declared their intent to join NATO, and jointly presented their official letters of application to Secretary General Jens Stoltenberg. This followed Russia's unprovoked invasion of Ukraine (which commenced in February 2022), a move that shattered peace in Europe and interrupted the rules-based international order. After taking part in the NATO Summit in Madrid in June, Finland and Sweden completed accession talks at NATO Headquarters in Brussels on 4 July, confirming their willingness and ability to meet the political, legal and military obligations and commitments of NATO membership. On 5 July, Allies signed the Accession Protocols for Finland and Sweden, which then became Invitees, attending NATO meetings under that title until all Allies ratify the Accession Protocols according to their national procedures. Following the ratification of Finland's Accession Protocol by all NATO Allies, the country was formally invited to accede to the Washington Treaty (also known as the North Atlantic Treaty), and joined the Alliance on 4 April 2023. By early 2024, Sweden’s Accession Protocol had also been ratified by all Allies according to their national procedures, after which the country became a full NATO member on 7 March 2024 .



  • expand timeline

    4 April 1949:  Signature of the North Atlantic Treaty by 12 founding members: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States. Article 10 of the Treaty provides the basis for NATO's "open door policy".

    18 February 1952:  Accession of Greece and Türkiye

    6 May 1955:  Accession of the Federal Republic of Germany

    30 May 1982:  Spain joins the Alliance (and the integrated military structure in 1998).

    October 1990:  With the reunification of Germany, the new German Länder in the East become part of NATO.

    January 1994:  At the Brussels Summit, Allied leaders reaffirm that NATO remains open to the membership of other European countries.

    28 September 1995:  Publication of NATO Study on Enlargement.

    8-9 July 1997:  At the Madrid Summit, three partner countries – Czechia, Hungary and Poland – are invited to start accession talks.

    12 March 1999:  Accession of Czechia, Hungary and Poland, bringing the Alliance to 19 members.

    23-25 April 1999:  Launch of the Membership Action Plan (MAP) at the Washington Summit. (Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia join the MAP.)

    14 May 2002:  NATO foreign ministers officially announce the participation of Croatia in the MAP at their meeting in Reykjavik, Iceland.

    May 2002:  President Leonid Kuchma announces Ukraine's goal of eventual NATO membership.

    21-22 November 2002:  At the Prague Summit, seven partner countries – Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia – are invited to start accession talks.

    26 March 2003:  Signing ceremony of the Accession Protocols of the seven invitees.

    29 March 2004:  Accession of Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia.

    21 April 2005:  Launch of the Intensified Dialogue on Ukraine's aspirations to NATO membership and related reforms, at an informal meeting of foreign ministers in Vilnius, Lithuania.

    21 September 2006:  NATO foreign ministers in New York announce the decision to offer an Intensified Dialogue to Georgia.

    28-29 November 2006:  At the Riga Summit, Allied leaders state that invitations will be extended to MAP countries that fulfil certain conditions.

    2-4 April 2008:  At the Bucharest Summit, Allied leaders invite Albania and Croatia to start accession talks; assure the country known at the time as the former Yugoslav Republic of Macedonia that it will be invited once a solution to the issue of the country's name has been reached with Greece; invite Bosnia and Herzegovina and Montenegro to start Intensified Dialogues; and agree that Georgia and Ukraine will become members in future.

    9 July 2008:  Accession Protocols for Albania and Croatia are signed. Allied foreign ministers agree that Georgia should develop an Annual National Programme under the auspices of the NATO-Georgia Commission.

    1 April 2009:  Accession of Albania and Croatia

    4 December 2009:  NATO foreign ministers invite Montenegro to join the MAP.

    22 April 2010:  NATO foreign ministers invite Bosnia and Herzegovina to join the MAP, authorising the North Atlantic Council to accept the country's first Annual National Programme only when the immovable property issue has been resolved.

    2 December 2015: NATO foreign ministers meeting in Brussels invite Montenegro to start accession talks to join the Alliance, while encouraging further progress on reforms, especially in the area of rule of law. In a statement on NATO's "open door" policy, ministers encourage Bosnia and Herzegovina to undertake the reforms necessary for the country to realise its Euro-Atlantic aspirations and to activate its participation in MAP. Ministers also reiterate their decisions at Bucharest and subsequent decisions concerning Georgia, welcoming the progress the country has made in coming closer to the Alliance and expressing their determination to intensify support for Georgia.

    19 May 2016: Allied ministers sign the Accession Protocol, following which Montenegro has 'Invitee' status and starts attending North Atlantic Council and other NATO meetings.

    5 June 2017: Accession of Montenegro

    11 July 2018: At the Brussels Summit, following the historic agreement between Athens and Skopje on the solution of the name issue, Allied leaders invite the government in Skopje to begin accession talks to join NATO. Full implementation of the agreement on the solution of the name issue is a condition for a successful conclusion of the accession process.

    5 December 2018: Allied foreign ministers decide that NATO is ready to accept the submission of Bosnia and Herzegovina's first Annual National Programme under the MAP.  The registration of immovable defence property to the state remains essential.

    6 February 2019: The Allies sign the Accession Protocol of the Republic of North Macedonia, following which the country takes part in NATO activities as an invitee.

    15 February 2019: The Republic of North Macedonia is officially recognised by its constitutional name, following the full implementation of the agreement between Athens and Skopje.

    27 March 2020: Accession of the Republic of North Macedonia

    18 May 2022: In a meeting with Secretary General Jens Stoltenberg at NATO Headquarters, Finland and Sweden simultaneously submit their official letters of application to join the Alliance.  

    29-30 June 2022: Heads of State and Government from Finland, Georgia, Sweden and Ukraine and the Defence Minister of Bosnia and Herzegovina take part in the NATO Summit in Madrid. NATO reaffirms its commitment to its "open door policy", formally inviting Finland and Sweden to become members of NATO, and promising to step up support for Bosnia and Herzegovina, Georgia and Ukraine.

    4 July 2022: Finland and Sweden complete accession talks at NATO Headquarters in Brussels. 

    5 July 2022: Allies sign the Accession Protocols for Finland and Sweden, making them official Invitees.

    4 April 2023: Accession of Finland

    7 March 2024: Accession of Sweden