NATO Treaty: what Article 5 requires and how joining and leaving work

The North Atlantic Treaty, also called the NATO Treaty or the Washington Treaty, obliges each member state to assist an ally that suffers an armed attack, but leaves each state free to decide what that assistance is (Article 5). A country joins only if all members agree (Article 10), and any member may leave by giving one year’s notice (Article 13).

The treaty was signed in Washington on 4 April 1949 and now has 32 member states. It is short, it contains no court or sanctions mechanism, and most decisions within NATO are political rather than legally enforceable. For businesses and individuals in the Netherlands, the legal consequences mainly arise in practice: in claims against NATO bodies and allied states, in damage caused by allied forces and in the way Dutch courts deal with international obligations.

What does the NATO Treaty say?

The treaty has fourteen articles. The core is the collective defence clause in Article 5, supported by consultation in Article 4 and by the rules on accession and withdrawal in Articles 10 and 13.

The founders kept the text short to leave room for national decisions. Articles 1 and 2 commit the parties to settle disputes peacefully. Article 3 requires them to maintain and develop their capacity to resist an armed attack, alone and together. Article 6 defines where Article 5 applies: the territory of the parties in Europe and North America, Turkey, islands under their jurisdiction in the North Atlantic north of the Tropic of Cancer, and their forces, vessels and aircraft in that area. Article 9 establishes the North Atlantic Council.

What does Article 5 require?

Article 5 treats an armed attack on one member state in Europe or North America as an attack on all. Each member must then assist the state attacked, but decides for itself which action it considers necessary.

The text refers to ‘such action as it deems necessary, including the use of armed force’. There is therefore no automatic obligation to intervene militarily. The type and scale of assistance remain a national decision, which has led to lasting legal and political debate about the strength of the guarantee. Article 5 also refers to the right of self-defence in Article 51 of the UN Charter, and any measures taken must be reported to the UN Security Council.

Article 5 has been invoked only once, after the attacks of 11 September 2001 on the United States. That led to NATO operations such as Eagle Assist and Active Endeavour. NATO later took the lead of the UN-mandated ISAF mission in Afghanistan, in which the Netherlands took part for many years.

What is Article 4 for?

Article 4 gives each member state the right to request consultations whenever it considers its territorial integrity, political independence or security threatened. It creates no obligation to act, but it is an important diplomatic tool.

Article 4 has been invoked several times, including by Turkey on more than one occasion, by Poland in 2014, and by eight eastern allies immediately after the Russian invasion of Ukraine in February 2022. Consultations under Article 4 often precede a joint political statement or an adjustment of NATO’s military posture.

How does the treaty relate to the UN Charter?

Under Article 7, the treaty does not affect the rights and obligations of the member states under the UN Charter. In case of conflict, obligations under the UN Charter prevail, as Article 103 of the Charter provides.

This is not merely theoretical. The NATO air campaign in Kosovo in 1999 took place without explicit authorisation of the UN Security Council, and its lawfulness under international law is still debated.

What is NATO’s legal status?

NATO is an international organisation with its own legal capacity. Its status, and that of its headquarters and forces, is set out in separate agreements concluded after the treaty itself.

The Ottawa Agreement of 1951 gives NATO legal capacity and regulates the privileges and immunities of the organisation and its international staff. The Paris Protocol of 1952 does the same for the allied military headquarters, such as SHAPE and the Joint Force Command in Brunssum. The NATO Status of Forces Agreement of 1951 (SOFA) regulates the legal position of the armed forces of one member state on the territory of another.

Under Article VII of the SOFA, criminal jurisdiction is shared. As a rule, the sending state has the primary right to prosecute offences committed in the performance of official duty and offences directed only against its own state, forces or personnel. The receiving state has the primary right in other cases, such as offences committed off duty. Under Article VIII, claims by third parties for damage caused by foreign forces are handled by the receiving state under its own law. The costs of an award are then usually shared: 75 percent for the sending state and 25 percent for the receiving state.

How does a country join NATO?

Under Article 10, the member states may, by unanimous agreement, invite any other European state that can further the principles of the treaty and contribute to security in the North Atlantic area. A single member state can therefore block or delay an accession.

In practice the procedure runs as follows:

• The candidate country submits a formal request to join.
• The allies assess whether it meets the political conditions, such as democracy and the rule of law, and the military and financial commitments.
• If all allies agree, accession talks follow and an accession protocol is signed.
• All existing member states ratify the protocol under their own constitutional procedures.
• The candidate deposits its instrument of accession with the government of the United States, which acts as depositary, and membership takes effect.

In the Netherlands, an accession protocol needs parliamentary approval under Article 91 of the Dutch Constitution. Parliament could in theory block an enlargement by withholding approval. That has not happened with any NATO enlargement so far.

What did the accession of Finland and Sweden show?

Finland and Sweden applied in May 2022, after the Russian invasion of Ukraine. Turkey and later Hungary delayed ratification, and the process was unblocked through political negotiations rather than any legal mechanism.

Turkey argued that both countries tolerated groups it regards as terrorist organisations. After a trilateral memorandum between Turkey, Finland and Sweden, Turkey gave its consent. Finland joined on 4 April 2023 as the 31st member. Sweden followed on 7 March 2024 as the 32nd member, after the Hungarian parliament had also approved. The treaty contains no procedure for a member state that attaches conditions to its approval that lie outside the treaty. The unanimity rule gives each ally that leverage.

Is there a right to join?

No. The open-door policy is a political commitment based on Article 10, not a legal right. A candidate cannot enforce accession if one member state refuses.

At the Bucharest Summit in 2008, the allies stated that Georgia and Ukraine would become members, without offering a Membership Action Plan. The treaty does not exclude states at war, but accession during an armed conflict would immediately bring Article 5 into play. That makes unanimous agreement politically very difficult while the conflict lasts.

Can a member state leave NATO?

Yes. Under Article 13, once the treaty had been in force for twenty years, any party may leave by giving notice of denunciation to the United States government. The withdrawal takes effect one year after that notice.

The treaty sets no further conditions. There is no procedure before the North Atlantic Council and no sanction. The drafters chose a simple exit so that no member would feel trapped. The twenty-year period ran from the entry into force of the treaty in 1949, so it has long expired. The exit route is therefore open to every member, including states that joined recently.

Did France leave NATO in 1966?

No. France withdrew from NATO’s integrated military command in 1966, under President de Gaulle, but remained a party to the treaty. It fully rejoined the military structure in 2009.

The French case shows that a member can reduce its military involvement without using Article 13. The political alliance and the treaty obligations, including Article 5, continued to apply to France throughout.

Could the United States leave without Congress?

That question has been debated in the United States for years. In December 2023, Congress passed a law providing that the President may not withdraw from the treaty without the advice and consent of two thirds of the Senate or an Act of Congress.

The provision is part of the National Defense Authorization Act for 2024. Whether a president could still act alone is disputed under US constitutional law. Under international law, a notice deposited under Article 13 would take effect after one year.

How does NATO take decisions?

The North Atlantic Council decides by consensus. There is no vote; a decision is taken when no member state objects.

Each member state therefore effectively has a veto, which explains why decisions can take a long time. Legally, NATO decisions are political commitments of the states that agree to them. No external court enforces them and there is no sanction for non-compliance.

Is the 5 percent spending target legally binding?

No. At the summit in The Hague on 25 June 2025, the allies committed to spend 5 percent of GDP annually on defence and security-related spending by 2035, but that commitment is political, not a legal obligation under the treaty.

Under The Hague Summit Declaration, at least 3.5 percent of GDP should go to core defence requirements and up to 1.5 percent to related spending, such as protecting critical infrastructure and civil preparedness. Member states submit annual plans, and the trajectory will be reviewed in 2029. Falling short leads to diplomatic pressure, not to legal consequences; the treaty basis is the general duty in Article 3.

How are disputes involving NATO resolved?

The treaty has no court and no formal procedure for disputes between member states. Disputes about contracts with NATO bodies or allied states do, however, regularly end up before national courts, including in the Netherlands.

Political differences between allies are resolved through diplomacy within the North Atlantic Council. Under Article XVI of the SOFA, disputes about that agreement are settled by negotiation, and otherwise by the Council. A supplier with an unpaid invoice has no NATO court to turn to and must go to national courts, where immunity becomes the central question.

What have the courts decided about immunity?

The fuel supply cases brought by Supreme Site Services show where the limits lie. The outcome depends on who is sued: a NATO headquarters with functional immunity, or individual member states acting in a commercial capacity.

The supplier had delivered fuel for the ISAF mission in Afghanistan and claimed payment from SHAPE and the Joint Force Command in Brunssum. In 2017, the District Court of Limburg held that the functional immunity of the NATO headquarters had to give way because there was no reasonable alternative means of resolving the dispute, which affected the right of access to a court under Article 6 of the European Convention on Human Rights (ECLI:NL:RBLIM:2017:1002). The Supreme Court of the Netherlands took a different view. On 24 December 2021 it upheld the immunity of SHAPE and the Joint Force Command, because the escrow arrangement set up for these payments offered a reasonable alternative means of resolving the dispute (ECLI:NL:HR:2021:1956).

In the meantime, the Court of Justice of the EU had answered questions referred by the Dutch Supreme Court (case C-186/19, 3 September 2020). It held that a claim of this kind can fall within the scope of civil and commercial matters under the Brussels I bis Regulation, unless it arises from the exercise of public powers. The supplier then sued ten member states directly. In 2025, the District Court of The Hague held that it had jurisdiction and that those states could not rely on immunity, because the fuel transactions were commercial in nature (ECLI:NL:RBDHA:2025:9705).

For companies that supply NATO bodies or allied forces, the lesson is practical. Check before you sign who the contracting party is, which law applies, which court or arbitration clause applies and whether an internal dispute mechanism exists. Those choices determine whether you can enforce payment later. Our article on international dispute resolution explains the options.

How do NATO obligations work in the Dutch legal order?

The treaty itself was approved by the Dutch parliament and is binding on the Netherlands. Political decisions of the North Atlantic Council, however, do not have direct effect for citizens and companies.

Under Article 93 of the Dutch Constitution, provisions of treaties and decisions of international organisations are binding on everyone once published, if their content makes them binding on all persons. Under Article 94, such provisions prevail over conflicting Dutch legislation. NATO decisions are addressed to the member states and are political in nature. They generally do not meet that test, so a citizen cannot rely on them before a Dutch court. The Constitution does not allow courts to review Acts of Parliament against the Constitution itself (Article 120), but they do apply directly effective treaty provisions, such as the rights in the European Convention on Human Rights.

Dutch courts are cautious in foreign and defence policy, because the government has broad discretion there. That restraint has limits where international law sets mandatory rules. In the case about the export of F-35 parts to Israel, brought by Oxfam Novib, PAX and The Rights Forum against the State, the Advocate General at the Supreme Court discussed how far that discretion goes (opinion of 29 November 2024, ECLI:NL:PHR:2024:1279). Where a Dutch public body is involved, our administrative law team can advise on the options.

Who pays for damage caused by allied forces in the Netherlands?

If foreign NATO forces cause damage in the Netherlands, you claim compensation from the Dutch State, not from the foreign state. The State handles the claim and recovers most of the cost from the sending state.

That follows from Article VIII of the SOFA. Claims by third parties for damage caused in the performance of official duty are handled by the receiving state as if its own forces had caused the damage. For damage caused by NATO motor vehicles, the Netherlands has a specific statute, the NATO Motor Vehicles Damage Compensation Act (Wet vergoeding van door NAVO-motorrijtuigen veroorzaakte schade), which gives the injured party a claim against the State. Damage caused by a member of a foreign force outside official duty follows a different route, in which the sending state may offer an ex gratia payment. Keep evidence of the damage and of the vehicle or unit involved, and file the claim promptly. Our liability lawyers can assist with such claims; personal injury claims are referred to a specialised injury lawyer.

Can cyberattacks and hybrid threats trigger Article 5?

They can. Since the Wales Summit in 2014, NATO has recognised that a serious cyberattack can lead to the invocation of Article 5, but the allies decide case by case whether an attack reaches that threshold.

The treaty speaks of an armed attack, a term taken from Article 51 of the UN Charter. There is no binding definition of when a cyberattack, sabotage of undersea cables or a disinformation campaign reaches that level. In practice, hybrid threats are more often handled through Article 4 consultations, national measures and EU instruments than through Article 5.

What can we do for you with NATO-related legal questions?

Our civil lawyers and international law team handle the practical side of NATO law for companies and individuals:

• We review supply contracts with NATO bodies or allied forces before you sign: contracting party, applicable law, forum and dispute clause.
• We assess whether a NATO headquarters or a member state can rely on immunity against your claim in a Dutch court.
• We prepare and file a claim against the Dutch State under Article VIII of the SOFA for damage caused by allied forces.
• We send a formal demand for payment and start proceedings in the competent Dutch court where immunity does not apply.
• We advise on how international obligations, such as SOFA rules, apply in the Dutch legal order under Articles 93 and 94 of the Constitution.

Summary

• Article 5 obliges each member to assist an ally under armed attack, but each state decides what that assistance is.
• Accession requires the agreement of all members (Article 10); any member can leave with one year’s notice (Article 13).
• NATO decisions and the 5 percent spending target are political commitments, not enforceable legal obligations.
• NATO headquarters enjoy functional immunity in Dutch courts, but member states acting commercially can be sued.
• Damage caused by allied forces in the Netherlands is claimed from the Dutch State under the SOFA.

Frequently asked questions