The North Atlantic Treaty Organization (NATO) is a transatlantic political-military alliance between countries in North America and Europe, formed for the purpose of protecting member states’ security and sovereignty.
Twelve North American and Western European democracies created the framework for the alliance in 1949 with the signing of the North Atlantic Treaty. They established a system of collective defense, in which an attack against one Ally is considered an attack against all. NATO now consists of 32 member nations, the result of 10 rounds of enlargement since its founding.
"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." - from Article 5 of the North Atlantic Treaty
What is Article 5?
Article 5 of the North Atlantic Treaty sets forth member states’ commitment to collective defense: an armed attack against one NATO Ally shall be considered an attack against them all.
Article 5 triggers an obligation for each Ally to provide assistance, though what this response looks like—and indeed whether an issue is even elevated to an Article 5 invocation—has been variable in the three quarters of a century since the Alliance’s founding.
Explaining Collective Defense
NATO Allies’ commitment to collective defense, as outlined in Article 5, is a critical tool in safeguarding the freedom and security of member states, providing stability and deterring aggression.
Article 5 requires unanimous consensus among NATO Allies to be invoked and specifically commits each member state to respond to an armed attack. What their response looks like can vary greatly among members, as Allies are given significant discretion in deciding what they deem necessary to restore and protect security. Article 5 is vague in this regard.
The use of the term “armed attack” is a direct reference to Article 51 of the United Nations Charter. The North Atlantic Treaty does not specifically define what constitutes an “armed attack.” Member states could, by consensus, invoke Article 5 in response to a range of incidents, from a large-scale cyber attack or hybrid attack to a traditional military invasion. Their response depends on political agreement and feasible military options.
In practice, Article 4 has typically served as the starting point for NATO operations and been used to respond to emergency situations. Article 4 calls for consultations among Allies on security matters, requires consensus, and initiates a response involving joint decisions or actions. Its original intent was to provide a way to elevate issues, ensure protection, and deter aggression.
Trigger and Response
Though NATO member states have threatened to trigger Article 5 multiple times throughout the history of the Alliance, in circumstances ranging from a major cyberattack on Albania (October 2022) and the Trump administration’s threat to seize Greenland (January 2026), Article 5 has only actually been triggered once: in response to the 9/11 attacks on the United States. While Allies adopted a resolution on September 12 that stated Article 5 would apply if the attack was determined to originate from abroad, it was not until October 2—after being briefed on investigations into the attacks—that all member states agreed to invoke Article 5.
These dynamics have sparked criticism and debate over Article 5, and provoked skepticism regarding the credibility of NATO’s security guarantee. As exemplified by NATO’s 9/11 response, triggering Article 5 can prove a drawn-out method of responding to crises, slowed by diplomatic efforts, behind-the-scenes politics, and the difficulty of coordinating a unified response. An adversary may take advantage of this delay to escalate and quickly achieve limited objectives.
Perhaps for these reasons, Allies have considered triggering Article 5 only in rare instances; rather, they have historically relied upon Article 4 as a response mechanism. First invoked in response to the 2003 Iraq War, Article 4 has since been triggered nine times, with responses typically involving some sort of military or security assistance.
The nine instances that have triggered Article 4 consultations can be categorized into two groups: assistance requested by Turkey (often in response to Syrian aggression) and responding to Russian aggression (requested by various NATO Allies).
Assisting Turkey
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(October 2003) Trigger: Due to threats from the Iraq War, Turkey requested Article 4 consultations. Response: NATO launched aircraft assistance.
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(June 2012) Trigger: To respond to Syria shooting down an unarmed Turkish jet over international waters, Turkey called for consultations. Response: NATO issued a condemnation statement.
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(October 2012) Trigger: In response to Syrian shelling that killed civilians, Turkey invoked Article 4, a de-escalation from their original pursuit of Article 5. Response: NATO deployed Patriot missiles.
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(July 2015) Trigger: Turkey requested support after terrorist attacks. Response: The invocation of Article 4 was used by Turkey to brief Allies on its response and resulted in a condemnation statement from NATO.
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(February 2020) Trigger: Syrian airstrikes backed by Russia killed Turkish soldiers. Response: NATO issued a condemnation statement and augmented Turkish air defenses.
In Response to Russian Aggression
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(March 2014) Trigger: Latvia, Lithuania, and Poland requested Article 4 consultations over Russia’s occupation of Crimea. Response: Romania, Bulgaria, and Turkey deployed forces in the Black Sea, and NATO Allies increased aid to the Ukrainian government.
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(February 2022) Trigger: In response to Russia’s full-scale invasion of Ukraine, Bulgaria, Czechia, Estonia, Latvia, Lithuania, Poland, Romania, and Slovakia jointly requested Article 4 consultations. Response: Allies increased defense postures on NATO’s eastern flank. The process for Finland and Sweden joining the Alliance was also accelerated.
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(September 2025) Trigger: In two separate instances, Poland requested consultations following Russian drone incursion into Polish territory, while Estonia requested consultations following Russian fighter jets entering Estonia airspace. Response: Both incidents resulted in a collective policy for aerial defense on NATO’s eastern flank, established with Operation Eastern Sentry.
Assessing Credibility and Future of NATO
While NATO possesses the mandate to respond to aggression against its member states, the credibility of its security guarantee and confidence in the institution as a whole will rest in large part on its ability to act decisively.
The type of emergency situations that could prompt member states to collectively trigger Article 5 have the potential to evolve quickly. Meanwhile, it can take days or even weeks for NATO to reach consensus and respond, as seen with the example of the 9/11 attacks.
Even small disagreements among Allies can slow coordination or push states to take individual, rather than collective, action. The time required to reach consensus risks giving aggressors a window to escalate attacks.
The credibility of NATO’s security commitments will rest on the Alliance’s ability to improve decision-making processes and coordination. As Allies face new challenges—an ever-expanding range of threats, weakening confidence, and questions surrounding the United States’ reliability as a member—they should focus on responding to potential Article 5 triggers in a timely and coordinated manner.
These challenges, potential threats, and NATO responses are examined more closely in the Defense, Emerging Technology, and Strategy Program’s recent report “Russian Threats to NATO’s Eastern Flank: Scenarios, Strategy, and Policy for European Security.”
Despite ongoing skepticism toward NATO, its commitments have proven durable thus far. Only by improving its ability to decisively uphold Article 5 commitments will NATO be able to continue fulfilling its purpose of defending the sovereignty of its members through both deterrence and coordinated response.
