In June, I interviewed Timothy Parsons, author of The Rule of Empires and a professor of history at Washington University in Saint Louis. Parsons' work shows that empires are economically destructive not only for the conquered but often for the conquerors, as well. Furthermore, although empires appear to be strong, they are more vulnerable to conquest than more decentralized societies. Parsons describes how empires sustain themselves by creating distinct identities between subject and ruler. However, they must also co-opt members of the conquered society to assist in ruling the territory. These two necessities make empires inherently unstable.