Only other time NATO has approved an Article 5 action was after 9/11. Won't force any other nation to participate - but will legally authorise military action by those who wish to do so.
Telegraph:
NATO will Declare War on ISIL
A former Supreme Allied Commander for Nato has predicted the military alliance will declare war on Isil and send up to 15,000 combat troops, writes Ben Farmer, Defence Correspondent.
Adm James Stavridis forecast the attacks would prompt France to invoke the alliance's article 5 collective self defence clause, which states an attack on one member is an attack on all. The clause was used for the first and only time so far after the 9/11 attacks and led to the Nato military campaign in Afghanistan.
Adm Stavridis said: "I believe Nato should declare this article 5 and enter the fight." Military action would start with special forces raids and more air strikes, and include more training for Iraqi and Kurdish Peshmerga forces. He said faced with attacks on three fronts it would soon become clear that Isil "are not 10 feet tall. Nato can do this".
Most of the ground forces fighting Isil would be Iraqi and Kurdish, he predicted, but Nato troops would still be needed. He said: "I think this will be a mission of Nato boots on the ground in the range of 10,000 to 15,000."