When has NATO invoked 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).

The North Atlantic Treaty Organization (NATO) has invoked Article 5 only once in its history.1

This occurred in 2001 following the September 11 attacks against the United States. The alliance determined that the terrorist acts, which targeted the World Trade Center and the Pentagon, constituted an armed attack, thereby triggering the principle of collective defense where an attack on one member is considered an attack on all.1

It is important to distinguish this from Article 4, which involves consultation among members when security is threatened; while Article 4 has been invoked on multiple occasions, it is not a prerequisite for the activation of Article 5.1