Deterrence is part objective and part subjective. The objective part is that adequate military preparedness could be critical in some important circumstances. The subjective is, in part, the perception by the potential enemy of the other side’s power, determination, and courage, as well as its estimate of the wartime effectiveness of the other side’s relative nuclear capability. If the aggressor believes that his opponent has the weapons and will use them, deterrence is likely to be effective. Therefore, a nation’s defense lies, in part, in the credibility of its threat to use nuclear force as a last resort, which means that the need remains for coherent and plausible policies for the use of nuclear weapons.