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, including in response to the situation in Syria and the Russian invasion of Ukraine. NATO has standing forces on active duty that contribute to the Alliance’s collective defence efforts on a permanent basis.