The concept at the heart of NATO's 1949 treaty is collective defense. Collective defense is a principle that establishes that an attack on one member of the alliance is considered an attack on all members, and they will come to each other's defense. This concept is embodied in Article 5 of the North Atlantic Treaty, which states:
"The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, 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..."
The purpose of collective defense in the NATO treaty is to deter potential aggressors by demonstrating the unity and resolve of the member states. It aims to provide a deterrent effect and ensure the security and defense of all member countries. The principle of collective defense remains a fundamental aspect of NATO's functioning, and any attack on a member state is treated as an attack on the entire alliance, triggering a collective response.