How does Article 5 of the North Atlantic Treaty establish collective defense?

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Flag of NATO featuring a white compass rose emblem with four lines radiating from the center on a blue background. — North Atlantic Treaty Organization Flag of the North Atlantic Treaty Organization (NATO).

Article 5 of the North Atlantic Treaty establishes collective defense by stipulating that an armed attack against one or more member states in Europe or North America is considered an attack against them all.12

Upon such an attack, each member agrees to exercise its right of individual or collective self-defense, as recognized by Article 51 of the United Nations Charter. Consequently, every ally commits to assisting the party or parties under attack by taking whatever action it deems necessary—including the use of armed force—to restore and maintain the security of the North Atlantic area. This principle of mutual defense serves as the core of the NATO alliance, ensuring that the security of each member is inextricably linked to the security of the entire group.21