The vicious Emperor Commodus is dead, after one cruelty too many. How should a successor be chosen? By election? The Senate chooses the capable bureaucrat Pertinax. By purchase? The Praetorians like Didius Julianus, or at least his money. By inheritance? Clodius Albinus is from an ancient family with imperial ties. By force? A major branch of the army backs the blunt Septimius Severus. By acclamation? The common...