Legally, a year’s notice under Article 13; practically, giving up the Article 5 pledge, allied air policing and allied troops, then paying for deterrence alone.
The legal exit
The North Atlantic Treaty of 1949 has one exit clause. Article 13 says that after the treaty has been in force for twenty years, which it has been since 1969, any member “may cease to be a Party one year after its notice of denunciation has been given to the Government of the United States of America.” Washington informs the others. No vote, no negotiation and no consent from the other allies are required.
No member has ever used it. France left NATO’s integrated military command in 1966, expelled allied headquarters from its soil and stayed in the treaty; it returned fully to the command in 2009. Greece pulled its forces out of the command in 1974 over Cyprus and returned in 1980. Spain held a referendum in 1986 and voted to stay. One ally has added a hurdle of its own: since December 2023, US law bars a president from withdrawing without two-thirds of senators present or an act of Congress.
What a small state actually gets
Article 5 is the famous part: an armed attack on one ally is treated as an attack on all, with each ally taking the action it “deems necessary”. For a small country the less famous parts matter almost as much, because they work every day, not only in war.
Air policing. Small allies often cannot afford to keep fighter jets ready around the clock, so others do it for them. Allied fighters have guarded the skies of Estonia, Latvia and Lithuania from Šiauliai since the three joined in 2004. Italy and Hungary police Slovenia’s airspace; Greece and Italy cover Albania, Montenegro and North Macedonia. When Slovakia retired its MiG-29s in 2022, Poland and Czechia agreed to guard its skies until its new F-16s arrived.
Allied troops on the ground. NATO keeps multinational forward land forces in nine eastern member states, from Finland to Bulgaria; four were added after Russia’s full-scale invasion of Ukraine in 2022 and the Finnish one in 2026. A small host gets a larger ally’s troops on its soil: Sweden leads the force in Finland, Spain the one in Slovakia, Italy the one in Bulgaria.
A share in common assets at a small price. NATO’s commonly funded budgets, which pay for headquarters, the command structure and shared infrastructure, came to about €4.6 billion in 2025. Each ally pays a share set by its gross national income. For 2026–27 Estonia’s share is 0.143 % and Montenegro’s 0.036 %; for Czechia 1.16 % and Slovakia 0.50 %. In exchange, the country plans its defense inside an alliance whose members together spent an estimated $1.4 trillion in 2025, in NATO’s constant 2021 dollars.
Membership also imposes duties. Under Article 3, each ally must maintain its own capacity to resist attack. Allies treat 2 % of GDP as a floor, and at The Hague in 2025 all except Spain agreed to raise core defense spending to 3.5 % by 2035. A small state that leaves keeps its own defense bill and loses everything bought with the shared one.
What standing alone costs
The best-known estimates, from the IISS and from Bruegel with the Kiel Institute, model Europe as a whole replacing the United States; none costs a single small state going it alone. What exists are real countries that defended themselves outside the alliance, or still do.
Finland, before it joined in 2023, spent about 1.4–1.5 % of GDP on defense, by NATO’s figures for 2014–2021. Finland also kept universal male conscription, a wartime army of 280,000 and a trained reserve of around 870,000–900,000, shelters for about 4.8 million people, and in 2021 chose 64 F-35A fighters for about €8.4 billion. After Russia’s full-scale invasion of Ukraine, Finland applied to join NATO in 2022 and became a member in April 2023; Sweden followed in March 2024.
Switzerland spends about 0.8 % of GDP and keeps a conscript militia of roughly 147,000 people. Until the end of 2020 it could not keep fighters on alert around the clock. It can spend this little because EU and NATO members surround it: an attacker would have to cross allied territory first. Austria, also neutral, spent 1.1 % in 2025 and joined the European Sky Shield air-defense initiative in 2023. Ireland spends about 0.2 %, the EU’s lowest, and has reportedly relied for decades on a confidential arrangement for British jets to intercept threats in its airspace.
Israel shows what self-reliance costs a small state with hostile neighbors. In 2016–2022 it spent 4.3–5.3 % of GDP on its military; in 2025, at war, 7.8 %. Even Israel does not stand alone: under a 2016 agreement the United States provides it with $3.8 billion of military aid a year.
The benchmarks point one way. A small country on an exposed frontier that left NATO would need Finnish-style conscription and reserves, Israeli-level spending, or a protector outside the treaty. A sheltered one could spend little, as Switzerland does, but only because its neighbors stay in alliances.
Why no one has left
The case for leaving usually rests on sovereignty, or on not wanting to be drawn into other powers’ wars; the figures above are the price of that independence. For an exposed small state, no alternative buys as much security per euro: on top of the defense budget it needs anyway, it pays a fraction of a percent of a shared budget and gains the backing of the world’s largest militaries. Leaving would mean paying more for less security, while Russia is at war in Europe. The two countries that changed status in recent years, Finland and Sweden, moved in.
The same logic applies within Europe. A small country is safest inside a large defensive union, and safer still when that union plans and buys as one. That is the case for a Europe that could defend itself as one.