First strike

In nuclear strategy, a first strike is a preemptive surprise attack employing overwhelming force. First strike capability is a country's ability to defeat another nuclear power by destroying its arsenal to the point where the attacking country can survive the weakened retaliation while the opposing side is left unable to continue war. The preferred methodology is to attack the opponent's strategic nuclear weapon facilities (missile silos, submarine bases, bomber airfields), command and control sites, and storage depots first. The strategy is called counterforce.

During the Cold War period, both superpowers, NATO and the Eastern Bloc, built massive nuclear arsenals, aimed, to a large extent, at each other. However, they were never used, as after a time, leaders on both sides of the Iron Curtain realized that global thermonuclear war would not be in either power's interest, as it would probably lead to the destruction of both sides, and possibly nuclear winter or other extinction level events. Therefore, at times, both sides refrained from deploying systems capable of unanswerable nuclear strikes against either side. However, in both nations, there were interests that benefited from the development and maintenance of first-strike weapons systems: what U.S. President Dwight Eisenhower termed the military-industrial complex; these forces encouraged the constant development of weapons systems of greater accuracy, power, and destruction. In addition, each side doubted the other side's commitment to not deploy first-strike weapons, or even in the event of their deployment, to not strike first. Some first-strike weapons were deployed; however like most nuclear weapons, they were never used.

...
Wikipedia