The Praetorian Guard, the personal guard force of the Roman emperors, has become a byword for the ability of a bodyguard to control, and ultimately to elevate or depose, the person whom it is supposed to guard. By the nature of its situation throughout the early and middle eras of the empire -- as the only military force in near the capital of an empire most of whose armies were dispersed among the frontiers -- it had the potential to control its imperial masters. Against a conspiracy or riot an Emperor could call upon the Praetorian Guard, but if the Guard itself turned disloyal the Emperor could have no direct recourse unless personally at the head of legions, which the for the first half of the empire's history the emperors seldom were.
This danger became most dramatically manifest toward the end of the second century, after the death (by assassination, not at the hands of the Praetorians) of Commodus, the son of Marcus Aurelius. In the wake of Commodus' ill-rule and death, the Senate chose a distinguished elder member, Pertinax, as Emperor. Within three months he had been overthrown and killed by the Praetorian Guard. At that point the Guard -- accustomed by tradition to ample gifts on the accession of a new emperor -- took this principle to its logical limit and put the throne up for auction to the highest bidder (Gibbon, pp. 89-90).
This spectacle sealed the enduring popular reputation of the Praetorian Guard, but it -- and indeed the whole career of the Guard -- was in fact an instance of a more general problem, faced potentially by all states, the problem of controlling its army. The remainder of this discussion will first outline the nature of this problem, and some solutions that various states have employed. We will then examine the risks posed to the Roman state by the Roman army, and the place of the Praetorian Guard in the response to this risk devised by Augustus Caesar. Finally we will trace the earl...