When Article 5 is triggered, each Ally is obliged to assist the attacked Ally or Allies by taking “such action as it deems necessary” to respond to the situation. This is an individual obligation placed on each Ally, to be taken forward in consultation and coordination with other Allies.

In an Article 5 situation, NATO plays a vital role in this consultation and coordination process, providing a unified response in support of the attacked Ally. Through NATO, the attacked Ally can identify their security needs and receive offers of assistance, ensuring that actions taken by NATO and Allies are synchronised. For example, following the 9/11 attacks, NATO Allies agreed eight measures to support the United States (see the next section). Coordination through NATO does not limit Allies from taking unilateral or bilateral actions as well.

It is up to individual Allies to determine how they will implement the mutual assistance obligation, so long as their efforts can be regarded as consistent with what is necessary “to restore and maintain the security of the North Atlantic area.” Action pursuant to Article 5 may or may not involve the use of armed force.

Importantly, Article 11 of the Treaty acknowledges and accepts that there may be constitutional limitations that impact how individual Allies fulfil their obligation under Article 5 (the deployment of armed forces abroad may, for instance, be subject to prior parliamentary approval or consultation in some countries).

During the drafting of Article 5 in the late 1940s, there was consensus on the principle of mutual assistance, but differing views on how this commitment would be implemented in practice. The European participants wanted to ensure that the United States would automatically deploy its armed forces to defend their territory should one of the signatories come under attack; the United States did not want to make such a specific pledge, and this is reflected in the more flexible wording of Article 5, which obliges Allies to provide assistance but does not specify the type or degree of assistance that they choose to provide.