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What obligations do NATO members have when another member is attacked under Article 5?

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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).

Under Article 5 of the North Atlantic Treaty, NATO members agree that an armed attack against one or more of them in Europe or North America is considered an attack against them all.1

When such an attack occurs, each member is obligated to assist the party or parties under attack by taking, individually and in concert with other members, whatever action they deem necessary to restore and maintain the security of the North Atlantic area. This commitment includes the potential use of armed force, exercised in accordance with the right of individual or collective self-defense recognized by Article 51 of the United Nations Charter.1

This collective defense principle is the cornerstone of the alliance, ensuring that the security of each member is inextricably linked to the security of the entire group.21