Since f′ is presumed positive, the denominator must be positive if Vr is to be maximized with R < 1; but the condition that the denominator be positive is precisely the condition that Pc must meet in order that player R still prefer joint no-attack. Thus, if both players have optimal adjustments with R < 1, those optimal values of R and B are also perforce consistent with joint preference at no-attack.

Thomas C. Schelling, The Strategy of Conflict, loc. 504