1960 ... putting the 'A' in M.A.D.
As Dr. Strangelove would have pointed out; the insane logic of Mutually Assured Destruction* relies on the 'Assurance' that if there is a 'sneak attack' the defending team will still have enough surviving nuclear weapon delivery systems to totally destroy the aggressor. In 1960 as the first USN Polaris Missile nuclear submarines became operational that assurance guarantee became heavily underlined. Because these subs were nuclear powered they could cruise submerged for months. They could not be found underneath the vastness of the ocean and were largely invulnerable to any first strike by the Soviets. So, no matter what the outcome of a topside duel of a aircraft and land based atomic war was; the submarines always had enough missiles and megatons to launch and flatten the Soviet Union.
At first, in the early sixties, the range of the SLBM's (Submarine Launched Ballistic Missiles) was only about 1,200 miles. US subs had to lurk in waters close to the USSR as shown by the map above. The accuracy of the subs solid fuel 'Polaris' missiles was not good enough for hitting precise military targets. Instead the 1 megaton warhead was targeted at large population centers. A 'Polaris' Submarine carried sixteen missiles. The Soviet Union test launched their first SLBM about 2 months after the USA. The 'Triad' of air, land and sea atomic annihilation was in place. 'Assurance' was complete.
* Pentagon planners would rather we use the word 'deterrence'
Note: the USSR began operating their missile carrying nuclear subs two months after the United States.
https://en.wikipedia.org/wiki/41_for_Freedom
List of USN 'Polaris' submarines