There is a difference between a balance of terror in which either side can obliterate the other and one in which both sides can do it no matter who strikes first. It is not the "balance"—the sheer equality or symmetry in the situation—that constitutes mutual deterrence; it is the stability of the balance. The balance is stable only when neither, in striking first, can destroy the other's ability to strike back.

DEFINE: Clarifies the distinction between a symmetric balance of terror and a stable one, where stability requires both sides retain second-strike capability.

Thomas C. Schelling, The Strategy of Conflict, loc. 529