D. Military alliances made for “collective security.”
The North Atlantic Treaty Organization is a military alliance currently consisting of 29 countries from North America and Europe, established in April 1949, after World War II. It was made to ensure collective defense and mutual protection in the case of any external party's armed attack and it is still operating.
The Warsaw Treaty Organization (or Warsaw Pact) was created several years later, in 1955, as a way to counterbalance to NATO. It was founded as a military alliance made for collective security or mutual defense, consisting of the following members: Soviet Union (which was given the command of the armed forces of the member states), Albania, Poland, Romania, Hungary, East Germany, Czechoslovakia, and Bulgaria. It was fully dissolved in 1991.
