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"Article 5 provides that if a NATO Ally is the victim of an armed attack, each and every other member of the Alliance will consider this act of violence as an armed attack against all members and will take the actions it deems necessary to assist the Ally attacked."

https://www.nato.int/cps/en/natohq/topics_110496.htm#:~:text=Article%205%20provides%20that%20if,to%20assist%20the%20Ally%20attacked.