What Is NATO’s Article 5?

The article, as the cornerstone of a treaty signed in 1949, establishes solidarity among member states and has been invoked only once.
Article 5 is the cornerstone of the North Atlantic Treaty Organization (NATO) and states that an attack on one member of NATO is an attack on all of its members. Despite its importance, NATO has only invoked Article 5 once in its history—in response to the terrorist attacks of September 11, 2001.