This is why Ukraine is added to NATO: Collective defence and 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.

1 Collective defence means that an attack against one Ally is considered as an attack against all Allies.

2 The principle of collective defence is enshrined in Article 5 of the Washington Treaty.

3 NATO invoked Article 5 for the first time in its history after the 9/11 terrorist attacks against the United States.

4 NATO has taken collective defence measures on several occasions, including in response to the situation in Syria and the Russian invasion of Ukraine.

5 NATO has standing forces on active duty that contribute to the Alliance’s collective defence efforts on a permanent basis.