This can be defined as our assured-destruction capability. It is important to understand that assured destruction is the very essence of the whole deterrence concept. We must possess an actual assured-destruction capability, and that capability also must be credible. The point is that a potential aggressor must believe that our assured-destruction capability is in fact actual, and that our will to use it in retaliation to an attack is in fact unwavering. When calculating the force required, we must be conservative in all our estimates of both a potential aggressor's capabilities and his intentions.
Security depends upon assuming a worst plausible case, and having the ability to cope with it.
That is what deterrence of nuclear aggression means. It means the certainty of suicide to the aggressor, not merely to his military forces, but to his society as a whole. This is a somewhat ambiguous term, since it could mean simply the ability of one nation to attack another nation with nuclear forces first. This is the sense in which it should be understood. Clearly, first-strike capability is an important strategic concept.