What does it take to join NATO?

Editor’s note (April 6th 2023): This explainer was updated following Finland’s accession to the alliance.

NATO IS THE world’s most successful military alliance, central both to the West’s success in the cold war and to securing the European order that followed. Russia’s invasion of Ukraine has given it a renewed sense of purpose after decades of doubt about its role in the post-cold-war world. It has expanded greatly over the decades, growing from the original 12 allies in 1949 to 31 today. The latest to join was Finland on April 4th 2023. Sweden is almost in. And Ukraine is knocking at the door. Why do countries want to join NATO, and what does it take to become a member?

The core attraction is Article 5 of the Washington treaty, NATO’s founding document, which sets out the mutual-defence promise: “The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all”. This is often taken as a guarantee to defend militarily an ally under attack; in fact, a member is committed only to “assist” and to take “such action as it deems necessary” to restore or maintain security in the North Atlantic region. This may—or may not—include armed force.

There are few treaty rules to joining the alliance. NATO does not have an EU-style acquis, the vast body of European legislation that new members must adopt into domestic law. Article 10 of the Washington treaty asserts that the allies may unanimously invite “any other European State in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area”. It does not define “European”. Nor does it demand particular standards of democracy and governance, or say what the members’ contribution ought to be. Membership is thus largely a matter of political discretion—above all the wishes of America, the biggest contributor to the alliance and its ultimate guarantor, which extends its nuclear deterrence across NATO.

Greece and Turkey, though ruled at various times by military juntas, have been members since 1952. But with the accession of post-Franco Spain in 1982, NATO membership became more entwined with the democratisation of Europe, especially after the collapse of the Soviet Union. Accession has thus developed into a more formal process with accepted norms. One is NATO’s “open-door” policy, whereby the alliance holds out the prospect of membership to all European countries willing and able to join. Others were the requirements set out in the Study on NATO Enlargement, a policy document from 1995. They included: a functioning democratic political system based on a market economy; the fair treatment of minorities; a commitment to the peaceful resolution of conflicts; the ability and willingness to make a military contribution to NATO; and civilian control over military forces. Poland, Hungary and the Czech Republic acceded in 1999. That year NATO launched a “membership action plan” (MAP) to help other hopefuls. For many countries NATO membership became, in effect, a step towards EU membership. NATO members have promised to spend 2% of GDP on defence, but this target is not binding, and most allies still fall short of it.

Two sets of problems have slowed NATO enlargement in recent years. One involves the tangled conflicts of the western Balkans. North Macedonia, which joined in 2020, did so only after the settlement of a drawn-out dispute with Greece over its official name; Bosnia and Herzegovina, riven by internal tensions, remains in the antechamber. An even bigger difficulty has been Russia’s hostility to NATO spreading into ex-Soviet territories, particularly Georgia and Ukraine. At the Bucharest summit in 2008, divided NATO allies agreed on an awkward compromise: the two countries were not formally admitted into MAP, but were told vaguely they “will become members of NATO”. Later that year Russia waged a short war against Georgia in support of the separatist regions of Abkhazia and South Ossetia. Similarly in Ukraine, in response to the Maidan revolution of 2014, Russia seized Crimea and fomented a separatist revolt that led to the creation of the breakaway regions of Donetsk and Luhansk.

Russia’s invasion of Ukraine in February 2022, supposedly in the name of fending off NATO’s further expansion, has caused a profound change of mindset in the West. Russia is again an acute threat; defence spending is increasing in Europe, notably in Germany; allies are pouring arms into Ukraine. The war spurred Finland and Sweden to apply for membership. Despite their tradition of non-alignment, both Finland and Sweden have been close to NATO, particularly since 2014—sending soldiers to Afghanistan, taking part in NATO exercises, sharing intelligence and joining meetings of the alliance.

Prospective members have to send a letter of intent to NATO and, assuming it approves, hold talks on a range of political, defence, legal and technical issues. NATO would then draw up accession protocols which can be signed by ministers, or ambassadors to NATO. Even if these preliminary steps can be taken quickly, in a few weeks, membership requires ratification by all existing NATO members, which can take months. Sweden’s application has been held up by Turkey and Hungary, both of which have been sliding towards authoritarianism. Turkey says Sweden harbours exiles, including members of the Kurdistan Workers’ Party (PKK), who support terrorism. It wants several people extradited; Sweden says it will act in accordance with the rule of law. Hungary, for its part, objects to Swedish criticism of its government. Allies hope Turkey will lift the block to Sweden’s membership, as it has done for Finland, after its presidential election in May. Hungary is unlikely to hold out on its own. NATO’s summit in Vilnius in July has thus become an informal deadline. Until then, Sweden has been offered interim assurances of assistance were it to be attacked, even if it is not formally covered by Article 5.

Ukraine says it is high time for NATO to act on its promise of membership. In 2019 it amended its constitution to enshrine its quest for membership, and asked for fast-track membership in September 2022. But despite the West’s sympathy, not to mention extensive military supplies, the door remains barred. The allies say the priority is to help Ukraine survive as an independent state, retake lost territory and end the war on favourable terms. Declaring that Ukraine’s destiny is to be part of the “Euro-Atlantic family”, NATO has promised a multi-year assistance programme to shift Ukraine’s military hardware and doctrine to NATO standards. The West says Ukraine must also continue internal reforms, notably the fight against corruption.

Even if Ukraine prevails in the war, Russia seems unlikely to give up its desire to reconquer it—at least for as long as Vladimir Putin remains leader. Many allies, especially in Eastern Europe, think NATO membership is the best way to guarantee any peace in Ukraine and prevent a future attack. Others worry that it will draw NATO into a direct war with Russia—rather than today’s indirect one—and greatly raise the risk of a nuclear conflict.

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