Mutual Assured Destruction
Mutual Assured Destruction (MAD) was a strategy shared between the Soviet Union and United States that analysts widely believed prevented the Cold War from turning into a nuclear war. Its fundamental assumption was that both sides had a sufficient number of protected and deliverable nuclear weapons such that no conceivable counterforce attack, by the other side, could destroy enough weapons to prevent devastating countervalue second strike on the side starting the war.
Since countervalue threatens civilians, MAD was sometimes called a "balance of terror", in the sense that two superpowers threatened terror to one another's population. Both weapons that could make a disarming counterforce attack plausible (e.g., large numbers of independently maneuvering reentry vehicles) or prevent one (e.g., effective ballistic missile defense and integrated air defense systems).
Ethical problems abounded since many would argue it was mutual state terror, but it appears, pragmatically, to have worked.
It is not known if this was unique between the U.S. and U.S.S.R., or might exist between other nuclear-armed powers, such as India and Pakistan.