For a time after World War II, America held the upper hand with regards to nuclear superiority. It used this threat of "massive retaliation" as a means to deter Soviet aggression. By the late 1950s, the Soviet Union had built up a convincing nuclear arsenal that could be delivered on the territory of the United States and Western Europe.
By the mid-1960s, unilateral deterrence gave way to "mutual deterrence," a situation of strategic stalemate. The superpowers would refrain from attacking each other because of the certainty of mutual assured destruction, better known as MAD. This theory is still a major part of the defense policies of the United States and Russia.
Both superpowers recognized that the first requirement of an effective deterrent was that it should survive or "ride out" a surprise "counterforce" targeted attack without being decimate — a task made difficult by the ever increasing numbers of accurate delivery systems, "penetration aids," and multiple warheads.
This led to the foundation of the nuclear triad, or use of three different types of delivery systems (bombers, missiles, and submarines) to assure that a second-strike capability existed able to cause massive destruction to the attacking nation.
Both the Strategic Arms Limitation Treaty (SALT) and Strategic Arms Reduction Treaty (START) treaties all reflected attempts by the superpowers to manage strategic nuclear developments in such a way as to stabilize mutual deterrence. Ballistic missile defenses were outlawed; "first strike" weapons were decommissioned; civil defense was discouraged. However, neither the U.S. nor the Soviet Union was comfortable basing their country's defense on deterrence.
The U.S. has explored various Nuclear Use Theories (NUTs) such as "counterforce", "countervailing" or "flexible response." However, the status quo of MAD remains. Current arms control efforts are aimed at finding a minimum level of mutual assured destruction.