Collective defense is a principle in international relations whereby a group of countries agree to defend each other against external attack. If any member of the group is attacked, the other members must defend it and consider an attack on one as an attack on all. As in Article 5 of the North Atlantic Treaty Organization (NATO), an armed attack on one or more NATO members is considered an attack on all members, and they agree to take action, including the use of armed force, to restore security to the area.

• North Atlantic Treaty Organization. (1949). North Atlantic Treaty. https://www.nato.int/cps/en/natolive/official_texts_17120.htm