The Praetorian Guard was the emperor's private body guard which was made up of 9 cohorts under Augustus who had created them. It is unclear if these cohorts were equal in size to a legionary cohort (480 men). Each cohort was commanded by its own tribune of equestrian rank. But the overall command of the Praetorian Guard was entrusted to two equestrian "Prefects" with equal powers.
During the reign of Tiberius, the Guard was housed in their own barracks ( Castra Praetoria ). Hitherto, they had been stationed in large cities in central Italy.
Under Caligula, the number of Praetorian cohorts was raised to 12. And they had excercised for the first time their potential power as "king-makers" by forcing Claudius to become emperor following the murder of Caligula. Claudius had given the soldiers a handsome donative, or monetary award, to ensue their loyalty to him. This was an ominous precendent which would only feed the greed of the Guard in times to come.
Where ever the emperor went, even when he left the city of Rome, he would always be escorted by cohorts of the Guard. His immediate family members would also be escorted by Praetorians.
The loyalty of the Guard would waver during the reign of Nero when they had abandoned him to his fate. It also wavered under Nero's successor, Galba, when they were seduced into supporting Otho. Otho had the Guard lynch Galba and they made him emperor. Otho had used the Praetorians in battle against Vitellius' German legions. Vitellius defeated them and Otho killed himself. Vitellius took revenge of the Guard for taking up arms against him by disbanding them and king their centurions. He replaced the ranks of the Guard with his own legionaires and auxiliaries and formed them into 16 cohorts each 1,000 men strong. The disbanded members of the Guard would form the backbone of the army a pretender to the throne named Vespasian. They were backed by the armies of the Eastern provinces who supported him. Vespasian's forces had defeated those of Vitellius'. Afterwards, they laid siege to Rome with Vitellius inside. The army was led by Otho's former Guardsmen and they attacked the Praetorian camp. Vitellius was captured and killed which ended the civil war of 69 known as the "Year of Four Emperors".
Vitellius' Guard was merged with members from Vespasian's legions. Vespasian had the Guard reduced to 9 cohorts and they remained intensely loyal to both him and his dynasty. His son, Titus (the future emperor), was made sole prefect of the Guard. Under Domitian's (Vespasian's second son's) reign, the Guard was raised to 10 cohorts and they saw action on the battle field against the Germans across the Rhine and the Dacians across the Danube. Their presence was required since the emperor oversaw these campaigns in person.
Under Trajan, the guard saw more action against the Dacians when the emperor had conquered their country (modern Romania) and annexed it. Trajan was a great general who conducted expansionist wars in person. The Guard's presence was therefore required. Trajan had made an adjunct to the Guard by creating the Imperial Horse Guard ( Equites Singulares Augusti ) which served as the emperor's cavalry escort.
After Trajan's death, the empire would experience no major wars until the reign of Marcus Aurelius, 34 years later. Marcus had a co-emperor name Verus, whom he had sent to conduct a campaign against the Parthian Empire on Rome's Eastern border. Verus had a contingent of the Guard with him. After a successful campaign, both Verus and the Guard returned to Rome bringing with them a terrible plague which caused the deaths of thousands (if not millions) throughout the empire. Verus would die of the plague leaving Marcus alone to rule for a short while. Marcus had to deal with German tribes who succeeded in invading northern Italy and pillaging towns along the way. Marcus went to modern Austria to conduct punitive expeditions in person against the tribes and of course the Guard was brought along with him. The Guard fought with distinction and on one occasion one of the prefects was killed in action.
Under Commodus, the Guard had resumed their original function as merely body guards to the emperor since he withdrew from active campaigning. The prefect of the Guard had the emperor killed and proclaimed Pertinax, who was in on the plot, emperor. Pertinax paid the guard a huge bonus but they had later lynched him. The most notorious act the Guard undertook in its history took place - they auctioned off the throne to the highest bidder, Julianus. When the new emperor failed to keep his promises to the Guard, he was killed.
The empire was plunged into civil war between competing generals for about 4 years. Septimius Severus was the eventual winner of the civil war. He had the Guard disbanded and replaced by own troops.
When Severus died in 211, his sons, Caracalla and Geta, succeeded him jointly. Caracalla murdered his brother which displeased the Guard very much. Caracalla was later murdered by the prefect who himself became emperor. But the Guard murdered him as well and they elevated a cousin of Caracalla's to the throne named Elagabalus. He was a mere boy who was an effete homosexual interested in a strange Syrian cult that worhshiped the sun. Due to his flamboyance, the Guard replaced him with a cousin of his named Severus Alexander. But he too would not survive the Guard. With his murder in 235, the Severan dynasty became extinct which was the signal for civil war.
For the next 50 years the empire would experience complete anarachy and civil war between generals who coveted the throne. Many would be killed by the Guard. Most emperors didn't live beyond a year.
During Diocletian's reign (284-305 A.D.), the Guard was said to have been reduced in number. He had created the Tetriarchy, or the rule of four emperors. The empire was split into Western and Eastern halves. Each half was ruled by a senior emperor with the title of "Augustus". And each "Augustus" had a junior emperor with the title of "Caesar". The Guard was split between the two Augusti. The system was designed to prevent civil war from flaring up again and it worked as long as Diocletian remained in office. When he stepped down the system deteriorated and civil war would return. The Guard in Rome had made its last emperor in the year 305 - Maxentius. Maxentius defended Rome against the emperor Constantine (who ruled Gaul, Britain, and Spain) and was defeated at the battle of the Milvian Bridge (312 A.D.). Constantine entered Rome in triumph as the sole emperor of the western empire. He had the Guard permanently disbanded and tore down their barracks leaving only three walls of it standing.
The equipment of the Guard on the battle field during the Principate (81-180 A.D.) was most likely identical in most respects to their legionary counterparts. While on duty inside the city of Rome, they usually wore ordinary tunics and togas with their swords and daggers hidden beneath the folds. The only way to tell a Praetorian Guardsman from an ordinary civilian (in the city) was to look at his feet on which he wore military boots.