Merchant, Soldier, Sage: A new history of Power is a brilliant economic history treatise by David Priestland, who does a fantastic job explaining how the change in power between 4 castes in all societies explains all human history.
Those castes are: merchant, soldier, sage, peasant. Priestland explains the major flashpoints in history through the lens of each caste's struggle for dominance, including uneasy alliances between them.
When one or other of these castes becomes too dominant, crisis usually ensues and it is replaced by another. The dictatorial and hierarchical rule of the soldier caste is destroyed by defeat in over-ambitious military action; the hidebound and ossified rule of the sage caste leads to revolutionary uprisings designed to widen participation in the state; the unconstrained dominance of the merchant caste leads to economic instability and inequality, fuelling social conflict and sparking revolution.
