The Praetorian Guard: Rome's Kingmakers
The Praetorian Guard killed four emperors, elevated at least five more to power, and constituted the single most politically destabilizing institution in Roman imperial history. This was not a design intention. Augustus established the Guard as a personal security force — a professional bodyguard organized on military lines and stationed near Rome — because the emperor needed reliable protection and the Republic’s tradition of civilian governance had made no provision for one. What Augustus created as a security measure, his successors inherited as a power center whose loyalty could be purchased, whose commanders accumulated enormous influence, and whose physical proximity to the emperor gave it an influence over succession that no amount of constitutional theorizing could override.
The Guard’s organization reflected its origins as a military unit. It consisted of nine to twelve cohorts — the number varied across emperors and periods — each of approximately 500 men, drawn from the Italian and provincial populations but paid at significantly higher rates than ordinary legionaries and serving shorter terms. Their camp, the Castra Praetoria on the northeastern edge of Rome, was constructed by Tiberius’s praetorian prefect Sejanus in 23 AD, concentrating forces that had previously been dispersed across the city and its vicinity into a single fortified installation. The concentration increased the Guard’s military coherence and simultaneously increased its political leverage: a concentrated body of armed men in the city was a political fact that every emperor had to manage.
Sejanus himself is the clearest case of what a sufficiently ambitious praetorian prefect could achieve. He accumulated influence over Tiberius during the emperor’s increasingly reclusive later years, manipulated the succession by eliminating potential rivals from the imperial family, and came close — very close, by most historical assessments — to seizing power himself before Tiberius had him arrested and executed in 31 AD. Sejanus’s career demonstrated that the praetorian prefecture was potentially the most powerful office in the empire precisely because it combined military command, physical access to the emperor, and administrative responsibilities that could be used to control the flow of information. Subsequent emperors were wary of the prefects in proportion to their historical literacy; some limited prefect tenure or appointed multiple prefects simultaneously as a check; none fully solved the structural problem Sejanus had exposed.
The Guard’s most naked exercise of political power came in 193 AD. The emperor Pertinax — a capable administrator elevated after the assassination of Commodus — had offended the Guard by reducing the donativum, the traditional payment made to soldiers on an emperor’s accession or special occasion, and attempting to impose stricter discipline on a body accustomed to favorable treatment. The Guard killed him after less than three months in office and then held an auction for the imperial purple — the Historia Augusta’s account, perhaps embellished but not implausible in its outlines — with the wealthiest bidder winning. The winner, Didius Julianus, lasted sixty-six days before the frontier armies marched on Rome and Septimius Severus had him executed. Severus subsequently disbanded the existing Guard and reformed it with soldiers from his own Danubian legions, a structural intervention that temporarily improved the situation by replacing a Guard with established corrupt habits with one whose personal loyalty was to the emperor rather than to its own traditions.
The political logic of the Guard’s interventions was consistent across its history: it acted when the emperor’s legitimacy was contested, when the succession was unclear, or when material interests — primarily pay and privileges — were threatened. It was not ideologically driven, not consistently republican or monarchical in its political preferences, not attached to any particular dynasty. It was an armed interest group whose preferences were primarily financial and whose leverage was proximity. When emperors were strong and succession was clear, the Guard was a security service. When emperors were weak or successions were disputed, the Guard became an actor rather than an instrument.
Constantine disbanded the Guard permanently in 312 AD after his victory over Maxentius, who had relied on the Guard’s support and whose defeat made them a liability. Constantine replaced them with scholae palatinae — new palace guard units recruited from Germanic peoples — and destroyed the Castra Praetoria whose walls had sheltered three centuries of political manipulation. The institution that Augustus had created as a personal security force had outlasted seventeen emperors before being abolished by one who understood that its structural position made it incompatible with stable imperial power. The solution — replacing Roman guards with Germanic ones who had no Roman political tradition and no Roman political ambitions — worked for a generation. It also contributed to the progressive Germanization of the late Roman military that had its own long-term consequences.