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Why did NATO invoke Article 5 after the September 11 attacks?

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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).

NATO invoked Article 5 following the September 11 attacks because the alliance’s core principle dictates that an armed attack against one or more member states in Europe or North America is considered an attack against them all.12

The 2001 terrorist attacks, which were organized by Osama bin Laden, resulted in the destruction of the World Trade Center in New York City and part of the Pentagon outside Washington, D.C., causing the deaths of approximately 3,000 people. By invoking Article 5, the member states exercised their right to collective self-defense as recognized by the United Nations Charter, committing to assist the United States in restoring and maintaining the security of the North Atlantic area. This remains the only time in the history of the North Atlantic Treaty Organization that this article has been activated.12