On the evening of September 11, 2001, the members of the North Atlantic Treaty Organization invoked that treaty's mutual defense guarantee for the first time in the article's five-decade existence.

NATO's decision to implement Article Five of the treaty, pledging that an attack on one ally would be treated as an attack on all, has been viewed by some observers the beginning of the Alliance's transformation.

Originally set up to fend off Soviet expansionism during the Cold War, NATO continues to define its role in the 21st century. NATO after 9/11 is the subject of today's Dateline.