Dubbed the alliance's "article of faith," the heart of Article 5 is conveyed in its opening sentence: "The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all."
Often dubbed the "most successful" defense or military alliance in the history of the world, NATO's mutual defense commitment has never been put to a real test. Some would say this very fact is the best measure of the alliance's success.
Article 5 has been invoked just once, in the weeks after the September 11, 2001, attacks on New York and Washington. It was a gesture of solidarity by America's NATO allies, and was not formally taken up by Washington before 2003 when the alliance became involved in Afghanistan.
The ambiguity of the circumstances surrounding the first-ever invocation of Article 5 reflects changes in the global environment in which NATO operates, 60 years after its creation and 20 years after the end of the Cold War.
Karl-Heinz Kamp, director of the research division of the NATO Defense College in Rome, notes the contrast between the current era and the days of the Cold War.
"In today's security environment it is not always sure when Article 5 applies," Kamp says. "In the past it was quite clear -- when the first Warsaw Pact soldier comes through the Fulda Gap and puts his foot on West German soil, then an attack is there and the Article 5 would apply."
Since the Soviet Union ceased to exist, the nature of the threat has changed, with Afghanistan being the most obvious case. Most, if not all, allies agree that an unchecked terrorist presence in the region constitutes a threat of the first order for their national security. As former German Defense Minister Peter Struck famously noted in 2002, his country's security is "also defended in the Hindu Kush."
In its first 15 years after the end of the Cold War, NATO proceeded from the assumption that no significant threat to it existed in the former Soviet space. But now, in the wake of last year's Russia-Georgia war, "that assumption has now been proven false," U.S. Army General John Craddock, NATO's highest military commander in Europe, told the U.S. Senate's Armed Services Committee on March 24.
However, most NATO allies -- including the United States -- still don't see Russia as a major source of danger. Most would also agree that outright invasion is no longer the weapon of choice for any conceivable enemy.
Kamp says today's threats are much more complex, better camouflaged, and increasingly ambiguous and difficult to counter, from cyberattacks to energy wars, Accordingly, the applicability of Article 5, in recent years, has increasingly become a matter of interpretation.
To be sure, the decisions on whether and how to react to aggression have always been political. This fact is inherent in the language of Article 5 itself. The allies pledge to respond to an attack by "taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area."
NATO's new, ex-Soviet allies fear some allies in Western Europe could use these ambiguities to drag their feet should a conflict erupt between Russia and some of its former subjects.
France is often considered one of the usual suspects in these Eastern nightmare scenarios -- a country that could feel tempted to protect its ties with Moscow at any cost.
But Francois Heisbourg, a special adviser at the Paris-based Foundation for Strategic Research, argues that France is likely to side with the new allies in any future debate on the modern relevance of Article 5.
"This is an issue on which the French are actually very close to the East European," Heisbourg says. The French have traditionally tended to underscore the importance of Article 5 -- to the discomfort, sometimes, of the Americans, who often tended to prefer to see NATO doing things all over the place, but not [necessarily] in an Article 5 configuration."
As Karl Heinz Kamp notes, the discussion at NATO's upcoming 60th anniversary summit will focus on exactly this conundrum -- what the alliance will need to do to make Article 5 and its "core mission" of mutual defense credible in a complex post-Cold War environment.
NATO: 60 Years In Eight Minutes
60 Years In Eight Minutes
In Washington in 1949, the North Atlantic Treaty Organization was born. RFE/RL looks at the dramatic and difficult moments in NATO history with rarely seen archive films and exclusive interviews. Play
NATO At 60 series:
Rolling With The Changes
End Of Expansion?
The Article Of Faith
Getting The Balance Right