Collective defence and Article 5

  • Last updated: 04 Jul. 2023 11:47

The principle of collective defence is at the very heart of NATO’s founding treaty. It remains a unique and enduring principle that binds its members together, committing them to protect each other and setting a spirit of solidarity within the Alliance.

Map of NATO members countries


  • Collective defence means that an attack against one Ally is considered as an attack against all Allies.
  • The principle of collective defence is enshrined in Article 5 of the North Atlantic Treaty.
  • NATO invoked Article 5 for the first and only time in its history after the 9/11 terrorist attacks against the United States.
  • NATO has taken collective defence measures on several occasions, including in response to the situation in Syria and the Russian invasion of Ukraine.
  • NATO has standing forces on active duty that contribute to the Alliance’s collective defence efforts on a permanent basis.



A cornerstone of the Alliance

Article 5

In 1949, the primary aim of the North Atlantic Treaty – NATO’s founding treaty – was to create a pact of mutual assistance to counter the risk that the Soviet Union would seek to extend its control of Eastern Europe to other parts of the continent.

Every participating country agreed that this form of solidarity was at the heart of the Treaty, effectively making Article 5 on collective defence a key component of the Alliance.

Article 5 provides that if a NATO Ally is the victim of an armed attack, each and every other member of the Alliance will consider this act of violence as an armed attack against all members and will take the actions it deems necessary to assist the Ally attacked.

Article 5

“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 and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognized by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.”

This article is complemented by Article 6, which stipulates:

Article 61

“For the purpose of Article 5, an armed attack on one or more of the Parties is deemed to include an armed attack:

  • on the territory of any of the Parties in Europe or North America, on the Algerian Departments of France 2, on the territory of Turkey or on the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer;
  • on the forces, vessels, or aircraft of any of the Parties, when in or over these territories or any other area in Europe in which occupation forces of any of the Parties were stationed on the date when the Treaty entered into force or the Mediterranean Sea or the North Atlantic area north of the Tropic of Cancer.”

The principle of providing assistance

With the invocation of Article 5, Allies can provide any form of assistance they deem necessary to respond to a situation. This is an individual obligation on each Ally and each Ally is responsible for determining what it deems necessary in the particular circumstances.

This assistance is taken forward in concert with other Allies. It is not necessarily military and depends on the material resources of each country. It is therefore left to the judgment of each individual member country to determine how it will contribute. Each country will consult with the other members, bearing in mind that the ultimate aim is to “to restore and maintain the security of the North Atlantic area”.

At the drafting of Article 5 in the late 1940s, there was consensus on the principle of mutual assistance, but fundamental disagreement on the modalities of implementing this commitment. The European participants wanted to ensure that the United States would automatically come to their assistance should one of the signatories come under attack; the United States did not want to make such a pledge and obtained that this be reflected in the wording of Article 5.


Invocation of Article 5

The 9/11 terrorist attacks

The United States was the object of brutal terrorist attacks on 11 September 2001. The Alliance's 1999 Strategic Concept had already identified terrorism as one of the risks affecting NATO’s security. The Alliance’s response to 9/11, however, saw NATO engage actively in the fight against terrorism, launch its first operations outside the Euro-Atlantic area and begin a far-reaching transformation of its capabilities. Moreover, it led NATO to invoke Article 5 of the North Atlantic Treaty for the very first time in its history.

An act of solidarity

On the evening of 12 September 2001, less than 24 hours after the attacks, the Allies invoked the principle of Article 5. Then NATO Secretary General Lord Robertson subsequently informed the Secretary-General of the United Nations of the Alliance's decision.

The North Atlantic Council – NATO’s principal political decision-making body – agreed that if it determined that the attack was directed from abroad against the United States, it would be regarded as an action covered by Article 5. On 2 October, once the Council had been briefed on the results of investigations into the 9/11 attacks, it determined that they were regarded as an action covered by Article 5.

By invoking Article 5, NATO members showed their solidarity toward the United States and condemned, in the strongest possible way, the terrorist attacks against the United States.

Taking action

After 9/11, there were consultations among the Allies and collective action was decided by the Council. The United States could also carry out independent actions, consistent with its rights and obligations under the United Nations Charter.

On 4 October, once it had been determined that the attacks came from abroad, NATO agreed on a package of eight measures to support the United States. On the request of the United States, it launched its first ever anti-terror operation – Eagle Assist – from mid-October 2001 to mid-May 2002. It consisted in seven NATO AWACS radar aircraft that helped patrol the skies over the United States; in total 830 crew members from 13 NATO countries flew over 360 sorties. This was the first time that NATO military assets were deployed in support of an Article 5 operation.

On 26 October, the Alliance launched its second counter-terrorism operation in response to the attacks on the United States, Operation Active Endeavour. Elements of NATO's Standing Naval Forces were sent to patrol the Eastern Mediterranean and monitor shipping to detect and deter terrorist activity, including illegal trafficking. In March 2004, the operation was expanded to include the entire Mediterranean.

The eight measures to support the United States, as agreed by NATO were:

  • to enhance intelligence-sharing and cooperation, both bilaterally and in appropriate NATO bodies, relating to the threats posed by terrorism and the actions to be taken against it;
  • to provide, individually or collectively, as appropriate and according to their capabilities, assistance to Allies and other countries which are or may be subject to increased terrorist threats as a result of their support for the campaign against terrorism;
  • to take necessary measures to provide increased security for facilities of the United States and other Allies on their territory;
  • to backfill selected Allied assets in NATO’s area of responsibility that are required to directly support operations against terrorism;
  • to provide blanket overflight clearances for the United States and other Allies’ aircraft, in accordance with the necessary air traffic arrangements and national procedures, for military flights related to operations against terrorism;
  • to provide access for the United States and other Allies to ports and airfields on the territory of NATO member countries for operations against terrorism, including for refuelling, in accordance with national procedures;
  • that the Alliance is ready to deploy elements of its Standing Naval Forces to the Eastern Mediterranean in order to provide a NATO presence and demonstrate resolve;
  • that the Alliance is similarly ready to deploy elements of its NATO Airborne Early Warning Force to support operations against terrorism.


Enhanced collective defence measures

Although NATO Allies have only invoked Article 5 once, they have coordinated collective defence measures on several occasions.

On the request of Türkiye, on three occasions, NATO has put collective defence measures in place:

  • in 1991 with the deployment of Patriot missiles during the Gulf War,
  • in 2003 with the agreement on a package of defensive measures and conduct of Operation Display Deterrence during the crisis in Iraq, and
  • in 2012 in response to the situation in Syria with the deployment of Patriot missiles.

Following Russia’s illegal annexation of Crimea in 2014 and the rise of security challenges from the south, including brutal attacks by ISIL and other terrorist groups across several continents, NATO implemented the biggest increase in collective defence since the Cold War. For instance, it tripled the size of the NATO Response Force (NRF), a highly ready and technologically advanced multinational force; established a 5,000-strong Spearhead Force within the NRF; and deployed multinational battlegroups in Estonia, Latvia, Lithuania and Poland. NATO also increased its presence in the southeast of the Alliance, centred on a multinational brigade in Romania. The Alliance further stepped up air policing over the Baltic and Black Sea areas and continues to develop key military capabilities, such as Joint Intelligence, Surveillance and Reconnaissance. At the 2016 Warsaw Summit, Allies recognised cyberspace as a new operational domain, to enable better protection of networks, missions and operations. At their meeting in November 2019, NATO Foreign Ministers agreed to recognise space as a new operational domain, to "allow NATO planners to make requests for Allies to provide capabilities and services, such as hours of satellite communications."

Following Russia’s full-scale invasion of Ukraine – which started in February 2022 – and in line with its defensive planning to protect all Allies, NATO is taking additional steps to further strengthen deterrence and defence across the Alliance. This includes the deployment of the NRF for the first time in a deterrence and defence role. Allies have placed thousands of additional forces at high readiness, ensuring that the NRF continues to have the speed, responsiveness and capability to defend NATO territory and populations. Moreover, at an extraordinary Summit on 24 March 2022, NATO Leaders agreed to significantly strengthen the Alliance’s longer-term deterrence and defence posture. They agreed to reinforce the existing battlegroups and to establish four more multinational battlegroups in Bulgaria, Hungary, Romania and Slovakia. This has brought the total number of multinational battlegroups to eight, effectively doubling the number of troops on the ground and extending NATO’s forward presence along the Alliance’s eastern flank – from the Baltic Sea in the north to the Black Sea in the south. 

At the 2022 Madrid Summit, Allies committed to further concrete measures, such as deploying additional in-place combat-ready forces on the eastern flank, to be scaled up from the existing battlegroups to brigade-size units where and when required, underpinned by rapidly available reinforcements, prepositioned equipment, and enhanced command and control. They also made initial offers to NATO’s new force model, which will strengthen and modernise the NATO Force Structure and will resource a new generation of military plans. All these steps, together with the release of the 2022 Strategic Concept, which identified Russia as “the most significant and direct threat to Allies’ security and to peace and stability in the Euro-Atlantic area” will substantially strengthen NATO’s deterrence and forward defences.


Standing forces

Collective defence measures are not solely event-driven. NATO has a number of standing forces on active duty that contribute to the Alliance’s collective defence efforts on a permanent basis. These include NATO’s standing maritime forces, which are ready to act when called upon. They perform different tasks ranging from exercises to operational missions, in peacetime and in periods of crisis and conflict.

Additionally, NATO has an integrated air and missile defence system to protect Alliance territory, populations and forces against any air or missile threat or attack. NATO also conducts several air policing missions, which are collective peacetime missions that enable NATO to detect, track and identify all violations and infringements of its airspace and to take appropriate action. As part of such missions, Allied fighter jets patrol the airspace of Allies who do not have fighter jets of their own. They run on a 24/7 basis, 365 days a year.


  1. Article 6 has been modified by Article 2 of the Protocol to the North Atlantic Treaty on the Accession of Greece and Türkiye.
  2. On January 16, 1963, the North Atlantic Council modified this Treaty in its decision C-R(63)2, point V, on the independence of the Algerian departments of France.