Which international organizations formed as defensive alliances?

Several international organizations were formed as defensive alliances, notably NATO (North Atlantic Treaty Organization) established in 1949 to provide collective defense against aggression, particularly from the Soviet Union. The Warsaw Pact, created in 1955, served as a counterbalance to NATO among Eastern Bloc countries during the Cold War. Additionally, the Collective Security Treaty Organization (CSTO), formed in 1992, aims to ensure mutual defense among its member states in the post-Soviet space.