The Cohortes Praetoriae were the regular military bodyguard of the Emperor, their mission to act as the guarantors of his security -- though all too frequently, the Guard proved fickle in the loyalty of their commanders and soldiers. While prone to violently unmake as well as make emperors based on personal considerations such as service benefits and monetary recompense, the Guard linked their fortunes to the survival of the imperial office. As for the Emperor himself, the Praetorians were loyal as long as he did not threaten their interests.
Ultimately, the partisan tendency of the Guard proved to be their downfall. In 306 CE, the Praetorians threw their weight behind Maxentius, acclaiming him Emperor despite the marginally more legitimate claim of the previous Emperor's son, Constantine. A civil war followed that was finally decided in 312 CE at the Milvian Bridge outside the city of Rome, where Maxentius drowned while fleeing after his Praetorian troops broke in battle. The victor Constantine, who became the first Christian emperor, exacted revenge on the Praetorians for their loyalty to his enemy by disbanding the Guard entirely. In their place as the Emperor's bodyguard, Constantine established the Scholae Palatinae, composed of his own trustworthy troops, mostly of Germanic origin.
The legacy of the Praetorian Guard, as elite but unscrupulous guardians of an imperial regime, survives into our own modern age as a paragon of military amorality. Not bound by ethics, the Guard and their commanders, the Praetorian Prefects, regularly abused their power and access to the emperor in order to extort concessions, to murder with impunity, and to repress dissent. For more than three hundred years, the corps of Praetorians protected even the most venal of rulers without question, so long as their greed and ambition were served.
The Praetorian Guard, as they have become popularly known, were formally organized by Octavian following his final victory over Marcus Antonius (Mark Antony). In a symbolic gesture, Octavian -- who was soon to receive the title Augustus ("The Revered One") and become the first Roman emperor -- combined the surviving cadres of Antony's and his own bodyguard troops, called in Latin cohortes praetoriae, into the structured arm we know as the Praetorian Guard.
The first credible example of bodyguard-type cohorts dates to 133 BCE, when Scipio Aemilianus organized 500 of his clients into a so-called "troop of friends." The practice of keeping a bodyguard unit, later termed a cohors praetoria, became standard as the Republic devolved into civil war. (The cohors praetoria was named for the praetorium, the central area of the military camp where the commander's tent had been pitched; thus, the praetorium was the camp headquarters, and its guardians, the praetorian cohort.)
According to the Roman historian Appian, the rival successors to Julius Caesar's autocratic legacy, Marcus Antonius and Octavianus, each formed for themselves several praetorian cohorts out of a volunteer cadre of Caesar's legionary veterans:
They dismissed from military service the soldiers who had served their full time except 8,000 who had asked to remain. These they took back and divided between themselves and formed them in praetorian cohorts. (Appian, Civil Wars, Bk V.3)
From this veteran host, Antonius established three cohorts, Octavianus between five and nine. From Antonian coinage, scholars have surmised that Antonius' praetorian cohorts accompanied the general on his ill-starred campaigns in the East. Coinage minted in the late 30's BCE celebrates not only his cohortes praetoriae but a mounted praetorian cohort, a cohors speculatorum.
The basis for Octavianus having at least five cohorts comes from Orosius, who records that these five praetorian units accompanied Octavianus during the climactic battle with Antonius' army at Actium in 31 BCE. Tacitus records the existence in 23 CE of nine cohorts of the newly-instituted Praetorian Guard, a figure further supported by contemporary inscriptions.
Suetonius writes that Augustus stationed only three cohorts of Praetorians within the city of Rome, and these troops were billeted individually among the populace, not concentrated in a camp. The other cohorts were maintained in towns throughout Italia.
In the early years of the Empire, Augustus wielded his kinglike authority as princeps delicately to avoid offending the conservative Roman political sensibility. His billeting policy kept the Praetorian power dispersed and so concealed, like a mailed fist in a velvet glove, inconspicuous and seemingly inoffensive.
