Herein, what is the purpose of a nuclear triad?
The purpose of having this three-branched nuclear capability is to significantly reduce the possibility that an enemy could destroy all of a nation's nuclear forces in a first-strike attack. This, in turn, ensures a credible threat of a second strike, and thus increases a nation's nuclear deterrence.
Subsequently, question is, why did the US and USSR build so many nuclear weapons? Nuclear Arms Race
Though the United States and the Soviet Union were tentative allies during World War II, their alliance soured after Nazi Germany surrendered in May 1945. To help discourage Soviet communist expansion, the United States built more atomic weaponry.
Considering this, what was the purpose of placing nuclear missiles in hardened targets and using the nuclear triad?
Missiles are dispersed in hardened silos to protect against attack and connected to an underground launch control center through a system of hardened cables.
Why did the arms race happen?
Known as the Cold War, this conflict began as a struggle for control over the conquered areas of Eastern Europe in the late 1940s and continued into the early 1990s. Initially, only the United States possessed atomic weapons, but in 1949 the Soviet Union exploded an atomic bomb and the arms race began.