Home / Defense and security / Articles / What is NATO and what does it cost to be a member?

Russia’s invasion of Ukraine in February 2022 marks the second time the two countries have warred in the last decade and the largest land war in Europe since World War II. While many countries have punished Russia with a wide range of sanctions, there are still underlying security concerns throughout Europe.

In response to this, the North Atlantic Treaty Organization (NATO) has moved military forces into countries around Ukraine, providing a common defense for its member countries while also playing a peacemaker and humanitarian role in the region.

What is NATO and why was it formed?

NATO is a military alliance composed of 31 member countries from North America and Europe.[1] The organization was formed in 1949 as a collective defense organization to counter the Soviet Union during the Cold War.

Its primary purpose today is to promote security and defense cooperation among its members while also serving as a forum for consultation and coordination on political and security issues.

What is Article V? 

Article V is a crucial provision within NATO’s founding treaty that states that an attack on one member country will be considered an attack on all member countries. It signifies the collective defense commitment among NATO members. When one member is attacked, all other NATO countries are expected to assist allies, including with military force.

NATO invoked Article V for the first and only time when it joined the US in the war in Afghanistan after the September 11, 2001, terrorist attack.

The US Congress still needs to approve the request to go to war even if Article V of NATO is invoked. Therefore, provisions within the NATO treaty do not supersede the power of the US Constitution.

Which countries are in NATO and how do countries join?

Any European state may join after being unanimously invited by NATO members. Since 1949, the alliance grew from 12 to 31 members, with Finland joining in 2023. Both Finland and Sweden were invited to join NATO at the Madrid Summit on June 29, 2022.

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Based on the NATO treaty, any European state may join after being unanimously invited by members.

Which countries want to join NATO?

Three countries have declared a desire to join the alliance: Bosnia and Herzegovina, Georgia, and Ukraine. Bosnia and Herzegovina have participated in a NATO program for aspiring members since 2010. Alliance members agreed in 2008 that Georgia and Ukraine would become NATO members in the future, though there is no timeline.

Sweden, an official NATO invitee since July 2022, joined as the 32nd member on March 7, 2024

How much does it cost to be in NATO?

Each member contributes to a common fund based on the size of its overall economy. NATO’s common fund budget — $3.7 billion for 2023 — covers administrative costs and collective military infrastructure. One example of a common fund project: housing US military vehicles, tanks, and artillery at a Polish Air Force Base. The warehouse costs $360 million.

The US is projected to cover roughly 16% of NATO’s 2023 budget following the accession of Finland in April.[2] However, NATO's common fund is small compared to the expected defense spending of each of its members.

In addition to contributions to the common fund, member countries are expected to sustain their own national defense and military spending. In 2006, an informal agreement between members promised at least 2% of each country’s gross domestic product (GDP) to national defense or military spending. A 2014 NATO summit formalized this 2% agreement following Russia’s forced annexation of Crimea from Ukraine.

In 2022, seven out of 30 members exceeded the 2%-of-GDP target for defense spending. Greece spent the highest percentage on defense at 3.54%. The US is second with 3.46%.[3]

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Historically, the US spends more on domestic defense than the rest of NATO. After adjusting for inflation, total defense spending for non-US NATO countries hit a recent low of $308.0 billion in 2016. It increased 14.7% to $353.4 billion in 2022.

US total domestic defense spending increased 4.0% from $790.0 billion to $821.8 billion during that same period.

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Does NATO have its own military?

The NATO Response Force is an on-call multinational force of 40,000 troops contributed by various members on a rotating basis. While the force has been used multiple times for humanitarian purposes, it was activated for deterrence and defense for the first time in February 2022 in response to Russia’s invasion of Ukraine. However, the activated troops did not participate directly in the conflict. Since Ukraine is not a NATO member, the troops were readied for military action if Russia attacked a nearby member.

In addition to the response force, NATO members' combined active military forces numbered 3.3 million as of 2022. The US military accounts for 40.6% of that number. The world’s largest active military, by total enlisted forces, is China’s 2 million people.

A bar chart depicting the 10 largest active duty military forces in the world, along with combined NATO forces, in descending order.

What is the US military’s presence in Europe?

In September 2021, 65,000 active-duty American troops were deployed in Europe. This force was 37% of all US troops stationed abroad at the time.

There are multiple US military bases in Europe, including in Germany, the United Kingdom, Italy, and Spain.

Where has NATO deployed in the past?

The alliance plays an active role worldwide in more ways than the collective defense of its members. In the early 1990s, NATO forces helped stabilize the Balkan region after the breakup of Yugoslavia. It later protected Kosovo when Serbia invaded the country. In 2011, NATO deployed to Libya during the overthrow of Libya’s government. The alliance also supported humanitarian efforts in Pakistan in 2005, Sudan in 2006, and the US after Hurricane Katrina.

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The World Factbook
Last updated
July 19, 2023
Defense expenditure of NATO countries (2014-2022)
Last updated
March 21, 2023

As of July 27, 2023.


Valid as from April 4, 2023 until December 31, 2024


Figures for 2022 are estimates, according to NATO.