Nuclear deterrent composition
The doctrine of mutual assured destruction (MAD) assumes that a nuclear deterrent force must be credible and survivable. That is, each deterrent force must survive a first strike with sufficient capability to effectively destroy the other country in a second strike. Therefore, a first strike would be suicidal for the launching country.
In the late 1940s and 1950s as the Cold War developed, the United States and Soviet Union pursued multiple delivery methods and platforms to deliver nuclear weapons. Three types of platforms proved most successful and are collectively called a "nuclear triad". These are air-delivered weapons (bombs or missiles), ballistic missile submarines (usually nuclear-powered and called SSBNs), and intercontinental ballistic missiles (ICBMs), usually deployed in land-based hardened missile silos or on vehicles.
Although not considered part of the deterrent forces, all of the nuclear powers deployed large numbers of tactical nuclear weapons in the Cold War. These could be delivered by virtually all platforms capable of delivering large conventional weapons.
During the 1970s there was growing concern that the combined conventional forces of the Soviet Union and the Warsaw Pact could overwhelm the forces of NATO. It seemed unthinkable to respond to a Soviet/Warsaw Pact incursion into Western Europe with strategic nuclear weapons, inviting a catastrophic exchange. Thus, technologies were developed to greatly reduce collateral damage while being effective against advancing conventional military forces. Some of these were low-yield neutron bombs, which were lethal to tank crews, especially with tanks massed in tight formation, while producing relatively little blast, thermal radiation, or radioactive fallout. Other technologies were so-called "suppressed radiation devices," which produced mostly blast with little radioactivity, making them much like conventional explosives, but with much more energy.