Sejanus: Why the Roman emperor's most trusted friend betrayed the imperial family

Sejanus was born in 20 BCE, possibly in the Etruscan town of Volsinii, although his exact birthplace is uncertain.

He came from a family of equestrian status, which, although not part of the senatorial class, held enough influence to allow its members access to the senior levels of Roman administration.

His father was Lucius Seius Strabo and served as Praetorian Prefect before becoming governor of Egypt, which gave the younger Sejanus a route into imperial service that most outsiders could never access.

Some later writers suggest that Sejanus may have had early ties to the household of Gaius Caesar, the adopted grandson of Augustus, though direct evidence for this connection is lacking.

When his father had departed for Egypt in AD 16, Sejanus stepped into the role of Praetorian Prefect alongside a colleague whose identity is not firmly established in sources that survive.

By the following year, after his colleague's disappearance from the historical record, Sejanus had apparently secured sole command of the Guard, becoming the first man to hold the position alone.

As he tightened his grip on this military post, he began to use it as a means of protection for the emperor and as a tool to gain political power.

From the beginning of Tiberius’ reign, Sejanus demonstrated a clear understanding of how proximity to the emperor could lead to direct control over imperial decisions.

In AD 23, he persuaded Tiberius to move the Praetorian cohorts from their scattered barracks into a single fortified camp, the Castra Praetoria, on the edge of the city.

Built just outside the Servian Wall, the Castra Praetoria concentrated all the cohorts in one location and allowed Sejanus to unify control over the only military force that was legally stationed in Italy, largely transforming him from a palace official into one of the most powerful figures in the empire. 

Soon after this military consolidation, Sejanus took further steps to keep Tiberius apart from political interference.

He gained increasing control over who could meet with the emperor and blocked much of the correspondence sent to Capri.

After the death of Drusus the Younger, Tiberius’ only legitimate son and heir, in the same year, Sejanus' role increased.

According to Tacitus and Cassius Dio, Sejanus had poisoned Drusus with the help of his wife Livilla, whom he had seduced and drawn into a conspiracy that removed the most direct obstacle to his desire for power.

Livilla herself died under unclear circumstances in the aftermath of Sejanus' fall, and later accounts claim that her mother, Antonia Minor, either executed her or forced her to commit suicide, though contemporary evidence is silent on the matter.

As public honours flooded in, Sejanus used his military position and his reputation for loyalty to overshadow the emperor in Rome.

Many senators, who had once been cautious, began to praise him openly, while others joined their families to his by organising political marriages.

He often used his position to remove rivals, whom he sometimes targeted openly and at other times accused of maiestas, the unclearly defined charge of treason that had become a preferred legal weapon under Tiberius.

Among those targeted were prominent senators such as Lucius Arruntius and Quintus Haterius, who faced either exile or constant suspicion.

To secure his influence further, Sejanus ensured that his authority extended into the imperial household.

He acted as the filter through which nearly all information passed between Rome and Capri.

When he presented himself as essential to the running of the empire, he had slowly replaced the emperor in the minds of many Roman elites as the true source of political power.

After Tiberius withdrew from the capital permanently in AD 26, Sejanus transformed from a powerful official into the effective ruler of Rome for a time.

As he controlled grain distribution, dictated security policy, and held sway over trials in the Senate, the emperor became a distant and silent figure.

At the same time, Sejanus deepened his ties to the imperial family when he requested permission to marry Livilla.

Although Tiberius denied the request, likely due to Sejanus’ equestrian background, he did not punish the prefect for asking, which many interpreted as a unspoken sign of favour.

Although some modern writers have speculated that Sejanus arranged a marriage alliance with Claudius' son, no ancient sources confirm that his daughter married into the imperial family.

Nevertheless, when he secured posts for his relatives, he extended his reach across both the imperial network and the city’s governing class.

Increasingly, he accused opponents of disloyalty, and once accusations began, few survived the trial.

His supporters filled governorships and magistracies across the empire, while others fell silent to avoid suspicion. In practical terms, he operated as an emperor in all but name.

For several years, Sejanus managed to present his growing authority as part of the emperor’s will.

Yet behind the outward display of loyalty, he exercised unlimited power. In AD 31, he shared the consulship with Tiberius and received honours that placed him close to imperial status.

While some later interpretations suggest he held proconsular imperium, ancient sources do not explicitly confirm this.

He received public honours unprecedented for someone outside the Julio-Claudian family, including some statues in the Forum.

Although no official Roman coinage survives bearing his image, some provincial coins may have carried it, though this remains discussed by scholars.

At this height of power, Sejanus began to govern as if the imperial throne awaited him.

His men controlled the Praetorian Guard, the courts followed his instructions, and grain ships docked at his discretion.

Treason trials, once rare, became instruments of political purge. Even allies of Tiberius, including his long-serving friend Asinius Gallus, vanished under uncertain charges.

Fear, rather than admiration, now secured his place in Rome.

According to Suetonius, informants, who filled the city, recorded careless comments and made up conspiracies that gave Sejanus grounds to destroy political threats.

With control over imperial appointments, he replaced reluctant officials with loyal subordinates, and those who resisted often found themselves accused, convicted, and executed before Tiberius even became aware of the charge.

Despite his public honours, Sejanus remained at risk from the one man whose favour kept him in power.

On 18 October AD 31, during a regular session of the Senate, a letter arrived from Tiberius.

At first, it praised Sejanus in vague terms, but by the end, it instructed the Senate to arrest him immediately.

The emperor had already installed Naevius Sutorius Macro as the new Praetorian Prefect, and with the Guard under new command, Sejanus had no means to defend himself.

According to several sources, Macro had apparently tricked Sejanus into coming to the Senate by promising a new round of honours, ensuring he appeared unaware and unprepared.

The Senate immediately turned against him: guards dragged him to prison, and by nightfall, he was strangled and his corpse thrown down the Gemonian stairs, as crowds attacked the body.

Qithin days, his statues fell, his name vanished from public inscriptions.

Soon after, Tiberius ordered the arrest and execution of Sejanus’ children and allies.

His daughter was still a virgin and was raped before her death to comply with a traditional interpretation of Roman law, which forbade the execution of virgins.

Former supporters now hurried to denounce him in letters and speeches. The purge spread through every level of Roman society, as friends became enemies and no prior loyalty offered safety.

Josephus later suggested that Antonia Minor had secretly warned Tiberius about Sejanus’ desire for power, though this claim is absent from Tacitus and Suetonius.

Modern scholarship has asked whether Sejanus acted alone or with the quiet approval of the emperor.

Some argue that Tiberius may have been weakened by age and isolation and had given power fully to others and had only stepped in when he feared being replaced.

Others believe that Tiberius had grown suspicious years earlier and deliberately allowed Sejanus to rise so that he could expose his entire network of supporters at once.

Ancient authors such as Tacitus wrote decades later and tended to describe Sejanus as a corrupt courtier, but their portrayals often mixed historical analysis with moral judgment.

Suetonius wrote during Hadrian's reign nearly a century later and echoed this condemnation and tended to use Sejanus as an example of equestrian desire that exceeded its proper rank.

Still, the essential facts remain clear: Sejanus held real authority over the capital for half a decade, and during that time, he changed what a non-imperial official could control.

No other Praetorian Prefect before him had become so powerful, and those who followed never forgot how quickly such power could vanish.

His rise exposed the weakness of imperial succession, and his fall demonstrated how the emperor, even in exile, could destroy a man who had once governed Rome in his name.

Sejanus betrayed the imperial family and attempted to replace it, and this attempt taught him that proximity to the throne in Rome rarely guaranteed survival.