NATO

The North Atlantic Treaty Organization (NATO), sometimes called North Atlantic Alliance, Atlantic Alliance or the Western Alliance, is an international organization for defence collaboration established in 1949, in support of the North Atlantic Treaty signed in Washington, DC, on April 4, 1949. Its other official name is the French language equivalent, l'Organisation du Traité de l'Atlantique Nord (OTAN).
The core provision of the treaty is Article V, 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. Consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognized by Article 51 of the Charter of the United Nations, 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. …