What is NATO? The alliance and its Article 5 promise

NATO, the North Atlantic Treaty Organization, is a military alliance of 32 countries, two in North America and 30 in Europe. It was founded in 1949, at the start of the Cold War, and its core is a promise in Article 5 of its treaty: an armed attack on one member will be treated as an attack on them all. NATO makes its decisions by consensus, keeps a joint military command, and has invoked Article 5 once, after the 11 September 2001 attacks.

In short

• NATO is a defensive alliance: members pledge to come to each other's aid if one is attacked.
• Article 5 binds each member to act, but lets each decide how, so it is a firm promise without an automatic war.
• The alliance grew from 12 founders in 1949 to 32 members by 2024, and decides everything by consensus, with no votes.

Why was NATO created?

After World War II, Western Europe was exhausted and the Soviet Union's armies stood in the middle of the continent. Communist governments took power across Eastern Europe, and in 1948 and 1949 the Soviets blockaded West Berlin. Western European governments wanted a guarantee that the United States would not retreat across the Atlantic as it had after World War I.

The answer was the North Atlantic Treaty, signed in Washington on 4 April 1949 by 12 countries: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States. It was short, 14 articles, and for the United States it was a break with a long tradition of avoiding peacetime alliances, as the State Department's historians note. When West Germany joined in 1955, the Soviet Union and its allies answered with their own alliance, the Warsaw Pact. The two blocs faced each other until the end of the Cold War.

What does Article 5 actually promise?

Article 5 is NATO's center. Its key sentence says that an armed attack against one or more members in Europe or North America "shall be considered an attack against them all", and that each will help by taking "such action as it deems necessary, including the use of armed force".

The promise is firm about whether members act and flexible about how. The comparison below sets out what the article commits members to and what it leaves out, based on the treaty text and NATO's own explanation.

Article 5 does	Article 5 does not
Treat an armed attack on one member as an attack on all	Require every member to send troops or go to war
Oblige each member to assist the one attacked	Act automatically: the attacked member must ask for or accept help
Cover attacks on members' territory in Europe and North America, under Article 6	Cover attacks outside the area Article 6 defines, such as on members' islands south of the Tropic of Cancer
Allow the use of armed force "as it deems necessary"	Replace national decisions; each government acts under its own laws
Sit within the UN Charter's right of self-defense	Override the UN Security Council's role

Members have also agreed at summits that a serious cyberattack, and attacks to, from or within space, could count as an armed attack. Article 4 is separate and lighter: any member that feels threatened can call for consultations, which doesn't imply military action.

When has Article 5 been used?

Once. On 12 September 2001, the day after the attacks on New York and Washington, the North Atlantic Council agreed that if the attack had been directed from abroad, it would count under Article 5, and on 2 October it confirmed that it had. The first use of NATO's defining promise was to defend the United States, the member it had been designed to bring to Europe's defense. NATO aircraft then patrolled US skies, and allies later joined the war in Afghanistan, where NATO led the International Security Assistance Force from 2003 to 2014.

NATO has also acted in crises that did not involve Article 5: air campaigns in Bosnia in 1995 and Kosovo in 1999, a peacekeeping force in Kosovo, and an air campaign over Libya in 2011 under a UN Security Council resolution. Those operations, outside members' territory, were among the most argued-over in its history.

How does NATO make decisions?

NATO has no vote. Every decision is taken by consensus, meaning that all members agree or at least do not object, so the smallest member has the same say as the largest. The main forum is the North Atlantic Council, where each member has an ambassador, and which also meets at the level of ministers and of heads of state and government at summits. A Secretary General, a senior civilian from a member country, chairs it and speaks for the alliance.

NATO itself owns little military force. Its integrated military command plans for defense and leads operations, but the troops, ships and aircraft belong to the members and are put under NATO command when governments agree. The top commander in Europe has always been an American officer.

Spending is decided nationally too. Allies agreed in 2014 to aim at spending 2% of their GDP on defense within a decade, and at The Hague on 25 June 2025 they committed to 5% of GDP a year by 2035, of which at least 3.5% for core defense and up to 1.5% for related security and resilience. According to NATO, European allies and Canada together spent 1.4% of their combined GDP on defense in 2014 and 2.3% in 2025.

How did NATO grow to 32 members?

Article 10 lets the members invite any other European state that can contribute to the security of the North Atlantic area, and every existing member must agree. The membership list shows how it grew.

Year	Who joined	Members after
1949	12 founders	12
1952	Greece, Türkiye	14
1955	West Germany	15
1982	Spain	16
1999	Czechia, Hungary, Poland	19
2004	Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, Slovenia	26
2009	Albania, Croatia	28
2017	Montenegro	29
2020	North Macedonia	30
2023	Finland	31
2024	Sweden	32

Finland and Sweden, long neutral, applied together in May 2022, after Russia's full-scale invasion of Ukraine. Finland joined on 4 April 2023, and Sweden on 7 March 2024, after Türkiye and Hungary had delayed their approval.

Still argued

Two arguments about NATO have run for decades. The first is enlargement after the Cold War. Its supporters argue that it let newly free democracies choose their own alliances and has kept them safe; critics, including some Western realist scholars as well as the Russian government, argue that it provoked Russia by bringing the alliance to its borders. The second is burden-sharing: US governments of both parties have long pressed European allies to spend more, while Europeans have pointed to contributions that spending figures miss. The evidence on the first depends heavily on counterfactuals, which is one reason the debate doesn't settle.

NATO is often confused with other bodies. It is not part of the European Union, though 23 of the EU's 27 members also belong to NATO. It is not the United Nations, whose Charter it cites as the legal basis for self-defense. And it is an alliance of democracies by its treaty's own words, which commit members to "democracy, individual liberty and the rule of law".

Check yourself

• Without looking back: what does Article 5 oblige a member to do, and what does it leave to each member?
• Why was Article 5's only use a surprise, given why NATO was created?
• How does NATO decide anything without ever voting?

Sources

• The North Atlantic Treaty — NATO, 1949
• Collective defence and Article 5 — NATO
• NATO member countries — NATO
• The Hague Summit Declaration — NATO, 2025
• Defence expenditures and NATO's 5% commitment — NATO, 2026
• North Atlantic Treaty Organization (NATO), 1949 — US Department of State, Office of the Historian