As Mao euphemistically remarked, revolutions are not dinner parties. Violence is to be expected when political regimes are overturned. But the violence that accompanied modern revolutions is remarkable for the fact that it targeted fellow revolutionaries almost as often as declared opponents. Why is this? In this essay, I suggest that the reason has to do with a specific feature of revolutions that abandon constitutional forms of political legitimacy. These revolutions, following the precedent of the French “revolutionary government” (1793–94) and Marx's model of a “revolution in permanence,” tend to base the authority of their governments on the fulfillment of revolutionary expectations. This creates a political culture in which authority derives from the power to define what these expectations are, and what “revolution” means (much like Hobbes's sovereign had the power to set the meaning of words). But revolutionary culture does not leave room for Rawlsian pluralism. “There can be no solution to the social problem but mine,” proclaims the revolutionary ideologue in Dostoyevsky's The Possessed, expressing the law of the Red Leviathan. Such a system does not allow for loyal opposition. Accordingly, the specter of counterrevolution always hovers above disagreements between fellow revolutionaries. The purge thus becomes the necessary method for settling ideological differences.