"The Parties agree that an armed attack against one or more of them ... shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking ... such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area."
This is notable for what it doesn't include: anything beyond a vague agreement to "assist" as individually deemed necessary. If Russia were to invade Poland the rest of NATO could fulfill their Article 5 obligation by protesting strenuously.
-- Article 5 of the North Atlantic Treaty https://www.nato.int/cps/en/natolive/official_texts_17120.ht...
This is notable for what it doesn't include: anything beyond a vague agreement to "assist" as individually deemed necessary. If Russia were to invade Poland the rest of NATO could fulfill their Article 5 obligation by protesting strenuously.