Why were hardened missile silos important for nuclear deterrence?

Hardened missile silos were critical to nuclear deterrence because they provided the protection necessary to maintain a secure second-strike capability. By constructing these tubular, reinforced concrete structures deep underground, military forces could "harden" their intercontinental ballistic missiles against the blast and shock of a nuclear detonation.1

This fortification was essential to the strategic calculus of the Cold War. For deterrence to be effective, a nation had to ensure that its nuclear forces would not be entirely eliminated by a preemptive first strike from an adversary. If a sufficient number of missiles survived an initial attack, the defending state retained the ability to launch a retaliatory strike, thereby upholding the principle of mutually assured destruction (MAD). By making land-based ICBMs difficult to destroy, hardened silos discouraged potential aggressors from initiating a conflict, as they could no longer guarantee a decisive victory without incurring an unacceptable retaliatory response.123