Back during Cold War, nuclear weapon became enigma for being great power, a status of quality for any state and an absolute guarantee for its security. But developing nuclear weapon is one thing; successfully delivering it to its target is another. In World War times the prime mode of carrying out strike missions was air force which relied on long range strategic bombers to carry out bombing missions under the air cover of agile fighter air crafts. These strategic bombers were prone to enemy air crafts as well as anti-air craft fire. For delivering an expensive & destructive tool like nuclear bomb, air superiority was crucial which in several cases was not possible to achieve due to symmetric nature of conflict after rise of Soviet Union. During cold war, advancement in Ballistic Missile technology revolutionized the nuclear domain. It introduced a land based credible & safe delivery mode to nuke hostile targets without risking lives of pilots. The addition of Land Based Nuclear Strike Capability along-side Air based nuclear strike capability introduced the Term Biad.
In mean time, more & better Strategic Bombers were also developed along-side Intercontinental Missiles (ICBMs). In Soviet-European strategic theater, the ground based nuclear launch sites & dedicated strategic squadrons of air forces of smaller nations like United Kingdom & France were prone to preemptive strikes from Soviet Union. The lack of strategic depth of European nations and idea of survivability from first wave of nuclear strikes further diverged the delivery mechanism of nuclear weapons. States were in consideration to develop nuclear delivery mechanism which could survive in first nuclear strike and provide assured second strike capability for maintaining deterrence. Though concepts like hardened missile silos and under-ground tunnel & bunkers for holding and operating strategic bomber fleet were developed, along-side concept of mobility was also applied where instead of fixed missile silos, mobile launch vehicles called Transporter Erector Launcher (TEL) were used to carry and launch nuclear missile. Idea was to bring increment in chances of survivability of strategic assets from enemy first counter force strategic strike. These concepts do work for geographically large states like Soviet Union and United States but had limited application for small European states. Hence assured second strike capability concept came into existence which was more precise than concept of second strike capability.
As a result, Sea Launched Ballistic Missiles were developed which were carried with in nuclear powered ballistic submarines (SSBN). These submarines theoretically had no need to refuel thus could stay submerged for very long duration of time & could travel around the Globe undetected. This unique feature gave birth to idea of Assured Second Strike Capability and also introduced a new term called Nuclear Triad. Nuclear Triad is the capability of a nuclear state to launch nuclear strike from land, air and sea. Land attack can be launched through cruise & ballistic missiles; air strike is conducted through Air Launched Cruise Missile (ALCM) or Gravity Bomb with nuclear payload; submarines in sea can utilize either Sea Launched Ballistic Missile (SLBM) or Sea Launched Cruise Missile (SLCM) for nuclear attack role. Though Surface vessels like destroyers can also be updated for carrying nuclear capable missiles but it is not a preferable way as unlike submarine, surface ships are more prone to enemy attack. This advance combination of Land, Air & Sea based nuclear strike capabilities once achieved by Nuclear armed state is considered to complete the Nuclear Triad of that state & with evolving threat environment it was considered crucial for keeping up deterrence among the hostile nations.
