While the Praetorian Guard was formally established under emperor Augustus, Sejanus introduced a number of reforms which saw the unit evolve beyond a mere bodyguard into a powerful and influential branch of the government involved in public security, civil administration, and ultimately political intercession; changes which would have a lasting impact on the course of the Principate.
During the 20s, Sejanus gradually accumulated power by consolidating his influence over Tiberius and eliminating potential political opponents, including the emperor's son, Julius Caesar Drusus. When Tiberius withdrew to Capri in 26, Sejanus was left in control of the entire state mechanism as de facto ruler of the empire. For a time the most influential and feared citizen of Rome, Sejanus suddenly fell from power the same year his career culminated with the consulship in 31. Amidst suspicions of conspiracy against the emperor, Sejanus was arrested and executed, along with his followers.
Strabo himself married into equally illustrious families. His first wife was Aelia, the daughter of Quintus Aelius Tubero, a marriage by which he allied himself with the more prestigious Aelian gens. By Aelia, he had one son, Lucius Seius Tubero, who became suffect consul in 18. After her death, he married Cosconia Lentuli Maligunensis Gallita, sister of Servius Cornelius Lentulus Maluginensis (suffect consul in 10) and Publius Cornelius Lentulus Scipio (suffect consul in 2), and half sister of Quintus Junius Blaesus (suffect consul in 10). With Cosconia, Strabo had one son, Lucius Seius, who was later adopted into the Aelian gens by Aelius Gallus, and by Roman custom became known as Lucius Aelius Seianus, or simply Sejanus.
The adoptive family of Sejanus counted two consuls among their family: the aforementioned Quintus Aelius Tubero (consul in 11 BC), and Sextus Aelius Catus (consul in 4), who was the father of Aelia Paetina, future wife of Claudius. Sejanus' uncle Junius Blaesus distinguished himself as a military commander who became proconsul of Africa in 21, and earned triumphal honors by crushing the rebellion of Tacfarinas.
According to the ancient historian Tacitus, Sejanus was also a former favourite of the wealthy Apicius, whose daughter may have been Sejanus' first wife Apicata. With Apicata, he had three children, two sons and one daughter: Strabo, Capito Aelianus and Junilla.
It is likely that Sejanus' father Strabo came to the attention of Augustus through his father's connection with Maecenas. Sometime after 2 BC, he was appointed prefect of the Praetorian Guard, one of the two most powerful positions a Roman knight could attain in the Empire. This office he carried on dutifully and without incident until the death of the emperor in 14. Little is known about the life Sejanus led prior to this date, but according to Tacitus, he accompanied Gaius Caesar, adopted grandson of Augustus, during his campaigns in Armenia in 1 BC. It was upon the accession of Tiberius in 14, that Sejanus was appointed prefect of the Praetorian Guard as the colleague of his father Strabo, and began his rise to prominence.
The Praetorian Guard was an elite unit of the Roman army formed by Augustus in 27 BC, with the specific function to serve as a bodyguard to the emperor and members of the imperial family. Much more than a guard however, the Praetorians also managed the day-to-day care of the city, such as general security and civil administration. Furthermore, their presence served as a constant reminder to the people and the Senate of the substantial armed force which served as the basis for the imperial power. Augustus was careful however to uphold the republican veneer of this regime, and only allowed nine cohorts to be formed (one less than in a normal Roman legion), which were inconspicuously scattered across various lodging houses in the city, and commanded by two prefects.
When Strabo was assigned to the governorship of Egypt in 15, Sejanus became the sole commander of the Praetorians and instigated reforms that helped shape the guard into a powerful tool of the principate. In 20 the scattered encampments inside the city were centralized into a single garrison just outside Rome and the number of cohorts was increased from nine to twelve, one of which now held the daily guard at the palace. The practice of joint leadership between two prefects was abandoned, and Sejanus himself appointed the centurions and tribunes. With these changes in effect, Sejanus now commanded the complete loyalty of a force of around 12,000 soldiers, all of which were at his immediate disposal. The facade of Augustus was no longer maintained, and Tiberius openly displayed the strength of the guard at parades.
In his capacity of Praetorian prefect Sejanus quickly became a trusted advisor to Tiberius. By 23 he already exerted a considerable influence over the decisions of the emperor, who referred to Sejanus as "my partner in my toils". By this time he had been raised to the rank of praetor, a position which was not normally granted to Romans of the equestrian class. A statue had been erected in his honor in the Theatre of Pompey, and in the Senate his followers were advanced with public offices and governorships. However this privileged position caused resentment among the senatorial class and the imperial family, in particular earning him the enmity of Julius Caesar Drusus, Tiberius' son.
The history of Sejanus and Drusus dated back to at least 15 AD. That year a mutiny had broken out among legions posted in Pannonia and Germania. While his adopted son Germanicus restored order in Germania, Tiberius' own son Drusus was sent to quell the uprising in Pannonia, accompanied by Sejanus and two Praetorian cohorts. In part due to what the soldiers believed to be bad omens, Drusus quickly managed to restore the stability in the army and publicly put the chief instigators to death. The camp was purged of mutineers by the Praetorians and the legions returned to the winter barracks. Despite this success, the following years witnessed a growing animosity between Drusus and Sejanus.
Since the accession of Tiberius, Drusus had been systematically groomed as the successor of his father, successfully commanding legions in Illyricum in 18, and sharing the consulship with Tiberius in 21. In practice however it was still Sejanus who was the second man in the empire, and he was ambitious to further expand his power. As early as 20, Sejanus had sought to solidify his connection with the imperial family by betrothing his daughter Junilla to the son of Claudius, Claudius Drusus. At the time the girl was only 4 years old but the marriage was nonetheless prevented when the boy accidentally died a few days later of asphyxiation.
When this failed it seems Sejanus turned his attention toward eliminating Drusus. By 23 the enmity between the two men had reached a critical point. During an argument Drusus had struck the prefect with his fist, and he openly lamented that "a stranger was invited to assist in the government while the emperor's son was alive". With Tiberius already in his sixties, there was a real possibility of Drusus succeeding his father in the near future. To secure his position Sejanus secretly plotted against him and succeeded in seducing his wife Livilla. With her as an accomplice Drusus was slowly poisoned and died of seemingly natural causes on September 13, 23.
The loss of his son was a major blow to Tiberius, both personally and politically. Over the years he had grown increasingly disillusioned with the position of princeps, and by sharing the tribunician powers with Drusus in 22 had prepared to relent some of his responsibilities in favour of his son. With these hopes now dashed, Tiberius left his administration more than ever in the care of Sejanus, and looked toward the sons of Germanicus as possible future heirs.
Germanicus himself had died in 19 in somewhat suspicious circumstances in Syria. Following his death, his wife Agrippina returned to Rome with their five children and became increasingly involved with a group of senators who opposed the growing power of Sejanus. Her relations with Tiberius became increasingly fraught as she made it clear that she believed that he was responsible for the death of Germanicus. The climate was further poisoned by the hatred that Tiberius' mother Livia Augusta felt for her, since Agrippina's ambition, to be the mother of emperors and thus Rome's first woman, was an open secret. To Sejanus personally, Agrippina's sons Nero, Drusus and Gaius Caligula were considered a direct threat to his power.
Meanwhile, Sejanus again attempted to marry into the Julio-Claudian family. Having divorced Apicata two years earlier he requested marriage with Livilla in 25, possibly with an eye towards placing himself, as an adopted Julian, in the position of a potential successor. The Emperor denied this request, warning Sejanus that he was in danger of overstepping his rank. Alarmed by this sudden denigration, Sejanus changed his plans and began to isolate Tiberius from Rome. By fueling his paranoia towards Agrippina and the Senate he induced the emperor to withdraw to the countryside of Campania, which he did in 26, and finally to the island of Capri, where he would spend the remainder of his life until his death in 37. Guarded by the Praetorians, Sejanus easily controlled all information that passed between Tiberius and the capital.
Despite the withdrawal of Tiberius from Rome's political scene, the presence of Livia seems to have checked Sejanus' overt power for a time. According to Tacitus, her death in 29 changed all that. Sejanus began a series of purge trials of senators and wealthy equestrians in the city, removing those capable of opposing his power as well as extending the imperial (and his own) treasury. Networks of spies and informers brought the victims to trial with false accusations of treason, and many chose suicide over the disgrace of being condemned and executed. Among those who perished were Asinius Gallus, a prominent senator and opponent of Tiberius who was linked to Agrippina's faction. Agrippina herself and two of her sons, Nero and Drusus were arrested and exiled in 30, and later starved to death in suspicious circumstances. Only Gaius (aka Caligula), as the last remaining son of Germanicus, managed to survive the purges of Sejanus by moving to Capri with Tiberius in 31.
Sejanus was so great a person by reason both of his excessive haughtiness and of his vast power, that, to put it briefly, he himself seemed to be the emperor and Tiberius a kind of island potentate, inasmuch as the latter spent his time on the island of Capreae.
Through years of crafty intrigues and indispensable service to the emperor, Sejanus had worked himself up to become the most powerful man in the empire. By the end of 31, he would be arrested, summarily executed and his body unceremoniously cast down the Gemonian stairs. Exactly what caused his sudden downfall is unclear: ancient historians disagree about the nature of his conspiracy, whether it was Tiberius or Sejanus who struck first, and in which order subsequent events transpired. Modern historians consider it unlikely that Sejanus plotted to seize the imperial power for himself and, if he had planned so at all, rather might have aimed at overthrowing Tiberius to serve as a regent to Tiberius Gemellus, son of Drusus, or possibly even Gaius Caligula. Unfortunately the relevant section pertaining to this period in the Annals of Tacitus has been lost. According to Josephus however, it was Antonia, the mother of Livilla, who finally alerted Tiberius to the growing threat Sejanus posed (possibly with information provided by Satrius Secundus), in a letter she dispatched to Capri in the care of her freedman Pallas.
Further details concerning Sejanus' fall are provided by Cassius Dio, writing nearly 200 years after the facts in his Roman History. It appears that, when Tiberius heard to which extent Sejanus had already usurped his authority in Rome, he immediately took steps to remove him from power, but realised that an outright condemnation could provoke Sejanus in attempting a coup against him. Instead he addressed a number of contradictory letters to the Senate, some of which praised Sejanus and his friends, others which suddenly denounced them. Alternatingly, Tiberius announced he would arrive in Rome the very next day, or claimed he was at the point of death. He stepped down as consul, thereby forcing Sejanus to do the same, and conferred an honorary priesthood upon Caligula, which rekindled popular support for the house of Germanicus. The ensuing confusion was successful in alienating Sejanus from many of his followers. With the intentions of the emperor no longer clear, it was now deemed a safer course of action at Rome to withdraw from overt support to Sejanus until the matter was clearly settled.
When it became clear to Tiberius that support for Sejanus was not as strong as he had feared, his next step was to choose Naevius Sutorius Macro, who had previously been prefect of the vigiles (Roman police and fire department), to replace Sejanus and accomplish his downfall. On October 18, 31, Sejanus was summoned to a Senate meeting by a letter from Tiberius, ostensibly to bestow the tribunician powers upon him. At dawn he entered the Senate, but while the letter was being read Macro assumed control of the Praetorian Guard, and members of the vigiles led by Graecinius Laco surrounded the building. The senators at first congratulated Sejanus, but when the letter, which first digressed into completely unrelated matters, suddenly denounced him and ordered his arrest, he was immediately surrounded and escorted to prison.
The same evening, the Senate convened at the Temple of Concord and summarily condemned Sejanus to death. He was led from prison, strangled and his body cast onto the Gemonian stairs, where the crowd tore it to pieces. Riots ensued in which mobs hunted down and killed anyone they could link to the terror regime of Sejanus. The Praetorians in turn resorted to looting when they were accused of having conspired with their former prefect. Following an issue of damnatio memoriae by the Senate, his statues were torn down and his name obliterated from all public records. On October 24, Sejanus' eldest son Strabo was arrested and executed. Upon learning of his death, Apicata committed suicide on October 26, after addressing a letter to Tiberius claiming that Drusus had been poisoned, with the complicity of Livilla. The accusations were further corroborated by confessions from Livilla's slaves, who admitted to having administered the poison to Drusus.
Enraged upon learning the truth, Tiberius soon ordered more executions. Livilla herself committed suicide or, as legend would have it, was forcibly starved to death by her own mother Antonia. The remaining children of Sejanus, Capito Aelianus and Junilla were executed in December of that year. According to ancient historians, because there was no precedent for the capital punishment of a virgin, Junilla was violated before her execution, with the rope around her neck. Their bodies were likewise thrown down the Gemonian stairs. At the beginning of the following year, damnatio memoriae was passed on Livilla.
Although Rome at first rejoiced at the demise of Sejanus, the city quickly plunged into more extensive trials, as Tiberius relentlessly persecuted all those who could in any way be tied to the schemes of Sejanus or had courted his friendship. The Senatorial ranks were decimated; the hardest hit were those families with political ties to the Julians. Even the imperial magistracy was not exempted from Tiberius' wrath. Arrests and executions were now supervised by Naevius Sutorius Macro. The political turmoil would continue until the death of Tiberius in 37, after which he was succeeded by Gaius Caligula.
The power Sejanus attained in his capacity as prefect proved Maecenas right in his prediction to Augustus that it was dangerous to allow one man to command the guard. Cassius Dio notes that after Sejanus, no other prefect except Gaius Fulvius Plautianus, who commanded the Guard under Septimius Severus, would rise to such influence. Nevertheless, following his death the guard began to play an increasingly ambitious and bloody role in the Empire. 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 Caligula was killed by conspirators from the senatorial class and from the Guard. The Praetorians placed Claudius on the throne, daring the Senate to oppose their decision. In the late 1st century, a calculated uprising of Praetorians against emperor Nerva, led by Casperius Aelianus, forced Nerva to adopt the more popular Trajan as his son and successor.
While it is unlikely Sejanus ever did intend to overthrow and replace Tiberius, later prefects of the Guard did aspire to become Emperor themselves. Upon the suicide of emperor Nero on 68, the Guard prefect Gaius Nymphidius Sabinus attempted to have himself declared emperor, on the pretense that he was the illegitimate son of Caligula. The attempt failed and Sabinus was killed by his own soldiers. In the early 3rd century, Plautianus was executed after a failed conspiracy against Septimius Severus. According to sources, the downfall of Plautianus was largely due to suspicion of Severus' son Caracalla, who was himself later murdered and replaced by his Praetorian prefect Marcus Opellius Macrinus.
Among the writers of the time which fell victim to the regime of Sejanus and its aftermath were the historians Aulus Cremutius Cordus, Velleius Paterculus and the poet Phaedrus. Cordus was brought to trial in 25 by Sejanus under accusations of treason. He was charged for having eulogized Brutus and spoken of Cassius as the last of the true Romans, which was considered an offence under the Lex Maiestas, and the Senate ordered the burning of his writings. His fall is elaborated upon by Seneca the Younger, in his letter to Cordus' daughter Marcia To Marcia, On Consolation. Seneca however tells us that her father most likely incurred Sejanus' displeasure for critizising him, because he had commissioned a statue of himself. We also know from this source that Cordus starved himself to death. Marcia was instrumental in saving her father's work, so that it could be published again under Caligula.
Phaedrus was suspected of having alluded to Sejanus in his Fables, and received some unknown punishment short of death (Cf. Fables I.1, I.2.24, and I.17). Velleius Paterculus was a historian and contemporary of Sejanus whose two-volume The Roman History details a history of Rome from the fall of Troy until the death of Livia Augusta in 29. In his work he praises both Tiberius and Sejanus, even defending the latter's high position in the government despite his equestrian rank. How much of Paterculus' writing is due to genuine admiration, prudence or fear remains an open question, but it has been conjectured that he was put to death as a friend of Sejanus.