A treaty of fourteen articles, and the clause invoked only once

Background

Article 5 of the North Atlantic Treaty states that an armed attack against one NATO member shall be considered an attack against all members, and triggers an obligation for each member to come to its assistance.
This assistance may or may not involve the use of armed force, and can include any action that Allies deem necessary to restore and maintain the security of the North Atlantic area.
On 4 April 1949, 12 countries from Europe and North America came together in Washington, D.C. to sign the North Atlantic Treaty, a founding treaty of only 14 articles and just over 1,000 words.
Article 5 is consistent with Article 51 of the United Nations Charter, which recognises the inherent right of individual or collective self-defence; within NATO this right becomes a mutual assistance obligation.
NATO invoked Article 5 for the first and only time in its history after the 9/11 terrorist attacks against the United States in 2001.
Article 6 of the Treaty imposes geographic limits, primarily confining the mutual assistance obligation to the North Atlantic area north of the Tropic of Cancer.
Article 4 provides for consultations between Allies whenever an Ally's territorial integrity, political independence or security is threatened, and such consultations are not a necessary precondition for action under Article 5.
The North Atlantic Council, NATO's principal political decision-making body, is convened immediately to discuss whether an attack is regarded as covered by Article 5.