Russia’s recent invasion of Ukraine marks the second time the two countries have warred in the last decade. Many countries have punished Russia with a wide range of sanctions in response to its aggression. The North Atlantic Treaty Organization (NATO) also moved military forces into countries around Ukraine. NATO provides a common defense for its member countries and plays a peacemaker and humanitarian role around the world.

What is NATO?

After World War II, the US, Canada, and 10 European countries formed NATO as a collective defense agreement. The agreement's primary purpose was to counter the power and influence of the then-Soviet Union and its allies. Article V of the NATO treaty states that an attack on one NATO member is an attack against all members. When attacked, member 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 Sept. 11, 2001, terrorist attack.

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 30 members, with North Macedonia joining in 2020.

NATO countries by year of admission

Year of admission Countries
1949 Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, United Kingdom, US
1952 Greece, Turkey
1955 Germany
1982 Spain
1999 Czech Republic, Hungary, Poland
2004 Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, Slovenia
2009 Albania, Croatia
2017 Montenegro
2020 North Macedonia

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

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

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 — $3.1 billion in 2021 — 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.

In 2021, the US was responsible for 16% of NATO’s budget, down from 22% in 2020. 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 2021, nine out of 30 members exceeded the 2%-of-GDP target for defense spending. Greece spent the highest percentage on defense at 3.8%. The US is second with 3.5%.

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Historically, the US spends more dollars on defense than the rest of NATO. Total defense spending for non-US NATO countries hit a recent low of $251.4 billion in 2014. It increased 17% to $301.8 billion in 2019. US defense spending increased 3% from $710 billion to $730 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 purposes for the first time in February 2022 in response to the Russian invasion of Ukraine. However, the activated troops did not participate directly in the conflict. Since Ukraine is not a member of NATO, the troops were readied for any military action necessary if Russia attacked a nearby NATO member.

In addition to the response force, NATO members' combined active military forces numbered 3.3 million as of 2019. US military accounts for 41% of that number. The world’s largest military, by total enlisted forces, is China's 2.1 million people.

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What is the US military’s presence in Europe?

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

There are multiple US military bases on the continent, 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 Sudan, Pakistan, and the US after Hurricane Katrina.

Learn more about US defense with our State of the Union in Numbers. Read more about US foreign aid to Afghanistan, Israel, and Ukraine.

The World Factbook
World Military Expenditures and Arms Transfers
NATO: Key Issues for the 117th Congress
Last updated
June 3, 2021
Congressional Research Service
Last updated
February 24, 2022
DoD Personnel, Workforce Reports & Publications

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