What sustains slavery, and why at critical junctures—the fall of the Roman Empire, the early modern expansion of plantation agriculture, the later phases of the industrial revolution, the totalitarian regimes of the 1930s—has it often expanded or contracted so rapidly? Why have elites sometimes been united, but sometimes violently divided, over the choice between free and servile labor? Why has slavery usually been ended by legal prohibition rather than voluntary abandonment? An extremely simple dual-equilibrium picture can illuminate how, when, and with whose support slavery is introduced or abolished. Internal divisions over slavery are likely to be most intense as a society approaches either of these “tipping points.” The most striking example, explored fleetingly here, is the US Civil War.