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Who can join NATO?

NATO says it has an ‘open door’ policy and any European country can join.

The only requirement is that they agree to further the principles of the Washington Treaty and contribute to the security of the North Atlantic area. This is set out in Article 10 of the founding 1949 North Atlantic Treaty (also referred to as the Washington Treaty).

Aspiring members are also expected to meet certain political, military and economic criteria. These are set out in the 1995 Study on Enlargement and include requirements such as a functioning democratic system, fair treatment of minority populations and a willingness to make a military contribution to NATO operations.

The UK was a founding member of NATO.

Can countries leave?

Yes. Article 13 of the Treaty gives Allies the right to leave should they wish to: “any Party may cease to be a Party one year after its notice of denunciation has been given”.

No country has left the Alliance.

Preparing to join: The Membership Action Plan (MAP)

Countries interested in joining the Alliance are usually invited to join what is called a Membership Action Plan (MAP).

Each MAP is tailored to the individual country and may involve political, legal, military, defence and security reforms.

Between 1999 and 2020 every new member that joined NATO had used a MAP. Aspiring member Bosnia and Herzegovina was invited to join the Membership Action Plan (MAP) in April 2010.

However, recent applicants Sweden and Finland did not use the MAP process.

Participation in the MAP does not guarantee membership of NATO.

How do countries join NATO?

Once an aspiring country has met all the requirements, or indicated it will be able to do so, it then informs NATO of its desire to accede to the Treaty. If all the members agree, NATO then invites the country to begin accession talks.

These are the first steps in joining. The remaining steps involve the signing of the Protocol to the North Atlantic Treaty on the accession of the invited country. This is known as the Accession Protocol.

Every Member State must sign and ratify the Accession Protocol. Once they have done so, they notify the US Government, which is the depositary for the Treaty. An invited country only join NATO when every member has deposited their instruments of accession with the US Government.

How long does the whole process take?

In practice, the application process can take several years if the aspiring country is required to complete any political, legal, military, defence or security reforms, or resolve any outstanding issues.

The ratification process usually takes about a year after the initial signing of the Accession Protocol by all the Member States.

Montenegro was invited to join the MAP in December 2009 and joined NATO in June 2017.

The Republic of Macedonia joined the MAP in 1999. After resolving the longstanding issue of its name with Greece, it joined NATO in March 2020.

Or it can occur very quickly if NATO members agree, as in the case of Finland. Finland applied to join NATO in May 2022 and became NATO’s 31st member in April 2023. Sweden joined NATO in March 2024.

What is the ratification process in the UK?

Since the passing of the Constitutional Reform and Governance Act 2010 (also known as CRAG), an Accession Protocol should be laid before Parliament for 21 sitting days’ scrutiny the Act. Provided there are no objections, the UK then deposits its instrument of ratification.

This was the case for the Accession Protocols of both Montenegro and the Republic of North Macedonia.

However, the Government opted to ratify the Protocols for Finland and Sweden without the 21-day requirement under CRAG having been met. The Government explained this was because of the urgent need to integrate the two countries into NATO as quickly as possible, and the lack of 21 sitting days of parliamentary time before summer recess. The Shadow Foreign Secretary, David Lammy, supported this approach.

The UK signed the Accession Protocols on 5 July. They were laid in Parliament on 6 July 2022 as Command Papers: CP 730 (Finland) and CP 731 (Sweden). The Protocols were deposited with the US State Department on 8 July 2022.

This briefing explains the above steps in greater detail.

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