This is known, in Latin as the Casus Foederis, Latin for "The Case for the Alliance." (Literally "The Case of the Alliance.") The Alliance includes accepting responsibility to go to war if the Alliance is thus triggered into action.
The NATO treaty members were triggered to war only once, so far. The United States invoked Article 5 after the 9/11 attacks. And NATO members dutifully participated in a number of military actions in Afghanistan as a result.
As I write this post, we are watching the death toll in the Paris attacks grow by the minute. And the fact is, France has every right, just as the USA did, to invoke Article 5 at this moment. They have been grievously attacked by an entity that calls itself a State. (I'm assuming, but I'm right, that this attack will eventually be deemed ISIS related.)
The term Casus Belli is Latin for "The Case for War." (Literally "The Case of the War.") This refers to an event that is used as a pretense for embarking on a war. Julius Caesar used the migration of the Helvetian people as a Casus Belli to invade and conquer Gaul. The United States of America used the explosion of the USS Maine as a Casus Belli to start to the Spanish-American War. (Granted, it is not at all clear that Spain had anything to do with the destruction of that ship.)
The fact is, the events of this day may very well be both a Casus Belli for France and a Casus Foederis. History may look upon today as a crucial juncture in the flow of this war.
We Americans must never forget the significant assistance France rendered us in our War of Independence. Did we repay that in WWI? Certainly. Did we repay that in WWII? Certainly. Is our debt to them cancelled? Never. We are friends forever. However they choose to respond to this attack, we must stand beside them.
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