North Atlantic Treaty Organization (NATO)

Collective Security: A type of coalition building strategy in which a group of nations agree not to attack each other and to defend each other against an attack from one of the others, should such an attack occur.

Cold War: The geopolitical tension between two superpowers, the Soviet Union and United States, after World War II. It's during this period that NATO was formed as part of containment strategy against potential aggression from Communist Bloc.

Article 5: The article within the North Atlantic Treaty that embodies the principle of collective defense. It states that if one NATO ally is attacked, then all NATO allies are considered to be under attack.