.. How did the United States' allies in NATO support
America's response to 9/11?|

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.

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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.