The balance of power
The balance of power is a process by which the power of competing groups of states tended toward a condition of equilibrium. Pursuit of a balance of power is a way of conducting foreign policy that is perhaps less prone to war than other types of policy because, instead of indiscriminately increasing their power, states increase it only moderately, so as not to provoke others; and instead of joining the strongest, they join the weaker side in order to assure balance. States in a balance system must, however, be ready to abide by constraints upon their behavior in order to ensure stability of the system.
Mutual Assured Destruction
An invasion of western Europe by the Soviet Union was considered unstoppable. The Soviet Union's conventional forces were significantly larger though technologically subordinate to NATO's. This threat led to the decision to deploy nuclear arms in Europe. The damage to the Soviet Union by a nuclear strike by the United States would be matched only by the damage to the United States by the Soviet Union. Each country would assure a sufficient nuclear force to respond to a first strike. Both had interest in preventing war, especially in Europe. This doctrine of being hostage of each other's behavior became known as the Mutual Assured Destruction or the more appropriate acronym MAD.
This in turn led to a change of NATO's policy, in which NATO's response would be dependent on the type of action by the Soviet Union. This became known as flexible response, a staged escalation of NATO's response to Soviet action. As long as the possibility of a nuclear strike remained at a credible level, the probability of a Soviet invasion remained relatively small.
In October 1957, the Soviet Union launched its first satellite. Before that, tests had taken place with an intercontinental ballistic missile or ICBM. In the United States, the fear grew that the US air bases would become possible primary targets in a surprise attack by the Soviet forces.
In the 1960's, US secretary of defense Robert McNamara was in the belief that as long as both super powers remained capable of doing unacceptable damage to the other, the military level of balance would be stabilizing.
Strategic Arms Reduction Talks (START) started in 1969 under president Richard M. Nixon, which didn't work out completely as expected. In 1972 the ABM Treaty was signed in which the two countries agreed to limit anti-ballistic missile systems and thus confirming the primacy of the offense. In June 1979 the SALT-II Treaty was signed, but didn't have any real value since this occurred at the same time as the Soviet invasion of Afghanistan.
A New Trend
An alternative to MAD was announced by president Ronald Reagan in March 1983. A defense system, capable of intercepting missiles would be established, known as the Strategic Defense Initiative (SDI) — popularly known as the Star Wars project — but the enormous costs involved caused a loss of interest by the politicians. The Strategic Arms Reduction Talks (START) didn't bring the expected arms reduction, but nevertheless a new trend in military thinking was set.