Article 5 and Strategic Reassurance. Atlantic Council Strategic Advisors Group, February 2010

As a declaration of strategic reassurance, Article 5 of the Washington Treaty could hardly be clearer: “an armed attack against one [of the allies]…
shall be considered an attack against them all and … if such an armed attack occurs, each of them … 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.”