Source: Shape.nato.int
NATO was born out of Article 5 of the North Atlantic Treaty, in which it was stated that “An armed attack against one or more member countries will be considered an aggression against all.”
North Atlantic Treaty Organization or “Atlantic Pact” consist in a mutual defense alliance, formed in response to Soviet aggression in Europe. The initial members included Belgium, Canada, Denmark, France, Great Britain, Iceland, Italy, Luxemburg, Netherlands, Norway, Portugal and United States.