Article 5 states: "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 that each party will assist the attacked party or parties by taking "such action as it deems necessary, including the use of armed force."

NATO explains that Article 5 states that an armed attack against one NATO member "shall be considered an attack against all members," and that it triggers an obligation for each member to come to the attacked ally's assistance.

The European Parliament briefing states that Article 5 of the 1949 North Atlantic Treaty is considered the cornerstone of the NATO Alliance and that when one NATO member falls victim to an armed attack, all other members must come to that state's assistance.

The Brennan Center quotes Article 5 as stating that an armed attack against one or more NATO members "shall be considered an attack against them all" and explains that Article 5 does not require any member to respond with military force, but permits each member to decide what action it deems necessary.

Indeed, the operative fulcrum upon which agreement rested was Article 5, a collective defense provision. Article 5 states that: 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-defense 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. Article 5 requires that Parties to the North Atlantic Treaty consider an attack against one or more of them to be an attack upon them all and assist the victim(s) either by providing assistance directly or in collaboration with other States.

In its explanation of NATO collective defence, NATO states in Ukrainian: "У Статті 5 Північноатлантичного договору йдеться, що збройний напад на одного з членів Альянсу вважатиметься нападом на них усіх." Translated to English: "Article 5 of the North Atlantic Treaty states that an armed attack against one member of the Alliance will be considered an attack against them all." The page then reproduces the Article 5 text beginning: "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..."

This article explains that Article 5 links "an armed attack against one [as] an attack against all," while also noting that each ally is free to take "such action as it deems necessary" and that Article 5 does not automatically imply military action.

The piece says the wording is vague: it states that an attack against one member "shall be considered an attack against them all," but each member only has an obligation to take "such action as it deems necessary, including the use of armed force."

The NATO public affairs description states: "Article 5 is the cornerstone of the Alliance. It means that an attack on one is an attack on all." It notes that "This article has been invoked only once in the 70-year history of the Alliance: in the aftermath of the September 11, 2001 terror attacks on the United States."

Wikipedia summarizes the key section of the treaty as committing each member state to consider an armed attack against one member state, in the areas defined by Article 6, to be an armed attack against them all.

A legal analysis of the treaty explains that Article 5 contains "the collective defense commitment" and that it "negates any possible suggestion of aggressive designs" by affirming that an armed attack on one or more parties is treated as an attack on all. The article situates Article 5 within the broader principles of the treaty and international law on collective self-defense.

At the heart of the alliance is Article 5, which states that an armed attack against one or more NATO members "shall be considered an attack against them all". If such an armed attack occurs, each member will assist the country or countries attacked by taking "such action as it deems necessary, including the use of armed force" to restore and maintain the security of the North Atlantic area.

The article states that Article 5 makes it a duty for every member state to help any ally that comes under armed attack, and notes that Article 5 was invoked once after the September 11 attacks.

Article 5 of the North Atlantic Treaty, NATO’s founding document, provides that an armed attack against one or more of its members in Europe or North America shall be considered an attack against them all. Consequently, each member agrees that, if such an armed attack occurs, it 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.

Article 5 of the North Atlantic Treaty codifies the principle of collective defence: an armed attack against one or more of the allies shall be considered an attack against them all. Once Article 5 is invoked, each ally will assist the attacked ally or allies by taking such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

A briefing on NATO explains: "Article 5 of the North Atlantic Treaty establishes collective defence: an armed attack against one or more Allies is considered an attack against all Allies." It further clarifies that each Ally "is obliged to assist the Party or Parties attacked" but that the form of assistance "is determined by each Ally in accordance with its constitutional procedures."