Explainer · World

NATO Article 5: What 'An Attack on One Is an Attack on All' Really Means

Last reviewed: September 26, 2026

Short answer: Article 5 of the North Atlantic Treaty says an armed attack on one or more NATO allies in Europe or North America “shall be considered an attack against them all.” Each ally then assists by taking “such action as it deems necessary, including the use of armed force.” It is a firm promise of mutual defense, but each country decides what action to take, through its own constitutional processes. NATO has invoked it once, after the 9/11 attacks.

The exact words

Article 5 reads, in full:

“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 recognised 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.”

The self-defense right it cites is Article 51 of the UN Charter, which preserves “the inherent right of individual or collective self-defence if an armed attack occurs.” More on the Council’s role: the UN Security Council and the veto.

What it requires, and what it leaves to each ally

• A shared obligation: NATO describes Article 5 as translating the right of self-defense into “a mutual assistance obligation.”
• Each ally chooses its response: the text commits each ally to take “such action as it deems necessary.” Armed force is included but not the only option.
• Constitutional processes: Article 11 says the treaty “shall be ratified and its provisions carried out by the Parties in accordance with their respective constitutional processes.” In the United States, the Constitution gives Congress the power “To declare War” and makes the President “Commander in Chief.” Treaties require the consent of two-thirds of the senators present.
• What counts as an attack: NATO says that must be judged case by case and is not limited to overt military strikes by a state. Purely domestic acts of terrorism, with no international element, do not trigger Article 5.

The one time it was used

NATO invoked Article 5 for the first and only time after al Qaeda’s attacks on the United States on September 11, 2001. The only invocation in NATO’s history came in defense of America.

The rest of the bargain: Articles 3 and 4

Article 3 obliges allies, “by means of continuous and effective self-help and mutual aid,” to “maintain and develop their individual and collective capacity to resist armed attack.” Article 4 provides for consultations whenever any ally believes its territorial integrity, political independence or security is threatened.

At The Hague summit in June 2025, allies committed to invest 5% of GDP a year on defense and security-related spending by 2035: at least 3.5% of GDP for core defense requirements and up to 1.5% for items such as protecting critical infrastructure and strengthening the defense industrial base. Allies agreed to submit annual plans, and the trajectory will be reviewed in 2029. The declaration ties the pledge to allies’ obligations “in accordance with Article 3 of the Washington Treaty.”

Membership and leaving

NATO has 32 member countries; Sweden became the 32nd on March 7, 2024. Under Article 13, after the treaty had been in force for twenty years, any party may leave “one year after its notice of denunciation has been given to the Government of the United States of America.”