During the rivalry between Rome and Carthage, Marcus Porcius Cato was famous for concluding every speech in the Senate – regardless of the subject matter – with the call that Carthage be destroyed. On the other side, Publius Scipio Nasica would counter that Carthage ought to be saved, because fear of the threat that it posed to Rome was the only thing that prevented the nobles and the plebs from descending into civil war. Nasica's advice, immortalized by Sallust and cited by Saint Augustine in City of God, became paradigmatic of the realization that human beings form and sharpen their identities as much by positive means as by reference to how they differ from others.

The history of political thought is filled with accounts of the mechanism captured by the old dictum “The enemy of my enemy is my friend,” and yet studies of group formation and collective action pay little explicit attention to this negative mechanism. In the only work of its kind, Wood identifies as “Sallust's Theorem” the realization that metus hostilis, the fear of enemies, “promotes internal social unity,” and proclaims it “a founding axiom of modern political thought.” Wood anticipates that modern readers may find Sallust's Theorem platitudinous. What is more, students of social movements may be tempted to ascribe the observation to Simmel, or to Sumner, who bequeathed to us the concepts of “in-group” and “out-group.”