Article 5 of the North Atlantic Treaty states that an armed attack against one NATO member shall be considered an attack against all members, and triggers an obligation for each member to come to its assistance.
From the nato.int website. It reads to me that if a country refuses to come to the assistance of a country legitimately invoking the article, the country is breaching the treaty.
will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
From the nato.int website. It reads to me that if a country refuses to come to the assistance of a country legitimately invoking the article, the country is breaching the treaty.
the issue is the exact wording is
https://www.nato.int/en/what-we-do/introduction-to-nato/collective-defence-and-article-5
“such action as it deems necessary”
assistance can mean many things, and can be very very minimal… eg purposefully ineffective sanctions would satisfy “will assist”