What NATO is and how it works

What the parties actually signed

The North Atlantic Treaty was signed at Washington on 4 April 1949 and came into force on 24 August 1949, once every signatory had deposited its ratification. It is short. Article 1 undertakes to settle disputes by peaceful means. Article 3 puts the obligation to build and keep military capacity on each party separately as well as jointly. Article 4 provides for consultation whenever any party considers its territorial integrity, political independence or security threatened. Article 9 establishes the Council on which each party sits.

Article 5 is the clause everybody names. An armed attack on one or more parties in Europe or North America is to be considered an attack on all, and each party will then assist the party attacked by taking such action as it deems necessary, including the use of armed force. The half that is less often quoted is the operative half: the choice of action is left to each government, and the measures taken are to be reported at once to the Security Council.

It has been invoked once. The Council met on the evening of 12 September 2001, less than a day after the attacks on the United States, and agreed that if it were determined that the attack had been directed from abroad, it would be regarded as an action covered by Article 5.

The line the treaty draws

Article 6 exists to say what an armed attack means for the purpose of Article 5, and it answers geographically: an attack on the territory of any party in Europe or North America, on the territory of Turkey, or on islands under a party's jurisdiction in the North Atlantic area north of the Tropic of Cancer, and an attack on the forces, vessels or aircraft of any party when in or over those territories, the Mediterranean Sea, or the North Atlantic area north of that same line.

The definition has been rewritten once and has contracted once. Article 2 of the Protocol on the accession of Greece and Turkey, signed on 22 October 1951, revised it, which is how Turkish territory came to be named there. The same text named the Algerian Departments of France, and on 16 January 1963 the Council noted that, so far as those departments were concerned, the relevant clauses had become inapplicable as from 3 July 1962.

Who is allowed to accede

Article 10 is the whole of the entry rule, and it is one sentence long. The parties may, by unanimous agreement, invite any other European State in a position to further the principles of the treaty and to contribute to the security of the North Atlantic area, and a state so invited becomes a party by depositing its instrument of accession with the Government of the United States. Two of the founding parties sit in North America and neither would qualify under the words that now govern accession: the door the treaty holds open is a European one. That same government keeps the treaty in its archives, and Article 14 makes the English and French texts equally authentic. Article 13 is the exit: after the treaty has been in force for twenty years, a party may cease to be a party one year after giving notice of denunciation to the Government of the United States.

Accession has happened in ten rounds. Greece and Türkiye acceded on 18 February 1952, the Federal Republic of Germany on 6 May 1955 and Spain on 30 May 1982. Czechia, Hungary and Poland followed on 12 March 1999, the first former members of the Warsaw Pact to do so; Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia on 29 March 2004; Albania and Croatia on 1 April 2009; Montenegro on 5 June 2017; North Macedonia on 27 March 2020; Finland on 4 April 2023; and Sweden on 7 March 2024. One enlargement of the treaty area involved no accession at all: German reunification in October 1990 brought the territory of the former East Germany inside it.

What holds an accession up

North Macedonia's waited on a name. The agreement struck between Athens and Skopje in 2018 settled the dispute, the country was officially recognised as the Republic of North Macedonia on 15 February 2019, and it acceded the following year.

Finland and Sweden moved together and arrived apart. Their letters of application were submitted on the same day, 18 May 2022; they were invited at the Madrid summit on 29 June, completed accession talks on 4 July and had their accession protocols signed on 5 July. Ratification of a protocol is then a separate act in every member state, done by each one's own procedure, and the eleven months between the two deposits are the measure of what that costs.

What membership does not settle

France withdrew from the integrated military structure in 1966 while remaining a party to the treaty and a participant in its political bodies. Foreign forces left French territory, French forces left allied commands, and both headquarters moved to Belgium: the political seat, in Paris since 1952, and Supreme Headquarters Allied Powers Europe, at Rocquencourt since 1951. France announced full participation in the structures again at the summit held at Strasbourg and Kehl in April 2009.

Iceland signed in 1949 with no armed forces and has never raised any. It keeps a coast guard, police forces, an air defence system and a voluntary expeditionary peacekeeping force, has held a bilateral defence agreement with the United States since 1951, and its airspace has been policed periodically by other members since 2008.

What the members have undertaken

Every decision is taken by consensus, in the Council and in every committee beneath it, which means that nothing is put to a vote and no member is outvoted. The undertaking given at The Hague on 25 June 2025 was adopted that way: to invest 5% of GDP a year by 2035, of which at least 3.5% goes to core defence requirements and up to 1.5% to critical infrastructure, networks, civil preparedness, innovation and the defence industrial base. The trajectory is to be reviewed in 2029.

Where the edges are argued

The alliance states that its door remains open on Article 10 terms and that no state outside it has a say in who is invited. Three partners have declared an aspiration to membership: Bosnia and Herzegovina, Georgia and Ukraine. At the Bucharest summit in April 2008 the allies agreed that Georgia and Ukraine would become members, and attached no date to it. Whether that door should open further is argued about outside the alliance and inside it, and this catalogue records the rule and the aspirations without settling either.

This is also not the European Union under another name. Austria, Cyprus, Ireland and Malta are in the Union and not here; Albania, Canada, Iceland, Montenegro, North Macedonia, Norway, Türkiye, the United Kingdom and the United States are here and not in the Union.