Collective defence - Article 5

The principle of collective defence is at the very heart of NATO’s founding treaty.

It remains a unique and enduring principle that binds its members together, committing 
them to protect each other and setting a spirit of solidarity within the Alliance.
Highlights
_Collective defence means that an attack against one Ally is considered as an attack against all Allies.
_The principle of collective defence is enshrined in Article 5 of the Washington Treaty.
_NATO invoked Article 5 for the first time in its history after the 9/11 terrorist attacks 
against the United States.
_NATO has taken collective defence measures on several occasions, for instance 
in response to the situation in Syria and in the wake of the Russia-Ukraine crisis.
_NATO has standing forces on active duty that contribute to the Alliance’s collective 
defence efforts on a permanent basis.
*More background information
A cornerstone of the Alliance
Invocation of Article 5
Enhanced collective defence measures
Standing forces
https://www.nato.int/cps/en/natohq/topics_110496.htm