The Praetorian Prefects

At the official inception of the Praetorian Guard by Octavian (Augustus) following his consolidation of power after Actium in 31 BCE, each cohors praetoria was an autonomous unit under the command of a military tribune -- a high-ranking member of  the ordo equester, the mercantile class of the Roman oligarchy.

In 2 BCE, Augustus consolidated the Praetorian cohorts under two overall commanders, the Praetorian Prefects, who exercised joint control.  While the command of individual cohorts remained the responsibility of a tribune, the Guard as a whole became the responsibility of the two Prefects.  Presumably the reason Augustus chose to divide such authority between two commanders was similar to the logic employed by the founders of the old Republic, who chose to elect two men as joint Consuls rather than vest too much executive power in one man.

The post of praefectus praetorio was often treated more as a political appointment than a military one.  While some Prefects might previously have held the post of praefectus urbi (commanding the Urban Cohorts), praefectus vigilum (commanding the Vigiles), or praefectus Aegypti, few Prefects had any combat command experience.  Until the time of Marcus Aurelius, Prefects were hardly ever called upon to lead their troops in the field.  In the case of Fuscus the Prefect of Domitian, the results he achieved in battle command were not indicative of expertise -- he misjudged the rough Dacian terrain, overextended his forces, and was slain on the field for his error.

On the other hand, Prefects chosen wisely by responsible Emperors were often dynamic and capable leaders; for example, the disciplined Prefect of Hadrian, Marcus Turbo, whom the historian Cassius Dio quotes as having said that a Praetorian Prefect should die on his feet.

For the Guard, the initial experiment in joint command worked, for a period of years.  Under the watchful eye of Augustus, the Praetorian Guard and its Prefects discharged their duties faithfully.  However, Augustus' successor Tiberius came to rely heavily on his Prefect, Lucius Aelius Seianus (called Sejanus).  With his father, Sejanus had served jointly as Prefect until 15 CE, when Sejanus became sole Prefect -- a dangerous precedent.

Sejanus insinuated himself into Tiberius' deepest confidence, then used that trust to manipulate the Emperor, enrich himself, and plot against the imperial family.  When Sejanus alienated the Emperor's son, Drusus, he felt himself threatened and connived with Drusus' wife to have the heir poisoned.  The plot succeeded, much to the detriment of the dynasty.  Seeing his path to the imperial throne open, Sejanus very nearly convinced the aging Tiberius to name him as heir, but in 31 CE, the Emperor discovered the extent of Sejanus' crimes.  The ambitious, ruthless Prefect was condemned and executed.

To secure the continued loyalty of the Praetorians after their Prefect's demise, Tiberius authorized a special payment (donativum) of 1,000 denarii, more than one year's pay to each Guardsman.  Though similar personal donatives had been gifted by past generals and rulers to their troops, this bonus -- a bribe, really -- was an ominous sign of where the true power lay behind the imperial throne, a reality that the Emperor and his Guard both recognized.