The deterrence concept requires that there be both conflict and common interest between the parties involved; it is as inapplicable to a situation of pure and complete antagonism of interest as it is to the case of pure and complete common interest. Between these extremes, deterring an ally and deterring an enemy differ only by degrees, and in fact we may have to develop a more coherent theory before we can even say in a meaningful way whether we have more in common with Russia or with Greece, relative to the conflicts between us.

DEFINE: Schelling clarifies that deterrence is only meaningful in mixed-motive situations, not in pure antagonism or pure harmony.

Thomas C. Schelling, The Strategy of Conflict, loc. 33