The Praetorian Guard was a special force of guards used by Roman Emperors. It was formed in by the emperor Augustus to help prevent assassins from reaching the emperor and murdering him as Brutus and his companions had murdered Julius Caesar. It was called by that name in imitation of the Praetoria Cohors, or select troop, which attended the person of the praetor or general of the Roman army. This cohort is said to have been first formed by Scipio Africanus out of the bravest troops, whom he exempted from all their duties except guarding his person.
The term “Praetorian” derived from the tent of the praetor, who was a commanding officer in the Roman army in the field. They were an elite recruitment of Roman citizens. It was a habit of many Roman generals to choose from the ranks a private force of soldiers to act as guards of the tent or the person. They consisted of both infantry and cavalry. In time, this cohort came to be known as the cohors praetoria. When Augustus became the first ruler of the Roman Empire in 27 BC, he decided such a formation was useful not only on the battlefield but in politics also. Thus, from the ranks of the legions throughout the provinces, Augustus recruited the Praetorian Guard.
Augustus divided the Praetorian Guard into nine cohorts of 500 soldiers each, just as with the regular legions. Only three of these cohorts were stationed in the capital; the others were dispersed in the adjacent towns of Italy. Before 2 BC each individual cohort was led by a tribune of equestrian rank. Afterwards, Augustus created two posts for overall command of the guard, the Praetorian Prefects. Quintus Ostorius Scapula and Publius Salvius Aper were the first Praetorian Prefects.
Augustus would be the sole emperor who could command the Praetorians’ complete loyalty. From his death, the Praetorians would serve whatever ends they believed were to their personal benefit. Through the machinations of their ambitious prefect, Lucius Aelius Sejanus, the Guard was brought from the Italian barracks into Rome itself. In 23, Sejanus convinced Tiberius to have the camp of the Praetorians built just outside of Rome. One of these cohorts held the daily guard at the imperial palace. Henceforth the entire Guard was at the disposal of the emperors, but the rulers were now equally at the mercy of the Praetorians. The reality of this was seen in 31 when Tiberius was forced to rely upon his own cohors praetoria against partisans of Sejanus.
The Praetorian Guard began to play an increasingly ambitious and bloody game in the Empire after the death of Sejanus, the prefect who conspired against the Emperor. With the right amount of money, or at will, they assassinated emperors, bullied their own prefects, or turned on the people of Rome. In 41 AD Caligula was killed by a detachement of the Praetorian Guard. Then the Praetorians placed Claudius on the throne, daring the Senate to oppose their decision.
But the Praetorian Guard was also a tough fighting elite unit. While campaigning, the Praetorians were the equal of any formation in the Roman Army. Seldom used in the early reigns, they were quite active by 69 AD. They fought well at the first battle of Bedriacum for Otho. Under Domitian and Trajan, the guard took part in wars from Dacia to Mesopotamia, while with Marcus Aurelius, years were spent on the Danubian frontier. Throughout the 3rd century, the Praetorians assisted the emperors in various campaigns.
The Praetorian Guard was definitively disbanded in 312 AD by Emperor Constantine, who defeated Maxentius. The soldiers were sent out to various corners of the Empire, and the Castra Praetoria was demolished. Although its name has become synonymous with intrigue, conspiracy, disloyalty and assassination, it could be argued that for the first two centuries of its existence the Praetorian Guard was, on the whole, a positive force in the Roman state. During this time it mostly removed cruel, weak, and unpopular emperors while generally supporting just, strong, and popular ones. By protecting these monarchs, thus extending their reigns, and also by keeping the disorders of the mobs of Rome and the intrigues of the Senate in line, the Guard helped give the empire a much needed stability that contributed to the period known as the Pax Romana.