Tiberius, becoming increasingly embittered with the position of Princeps, begins to depend more and more upon the limited secretariat left to him by Augustus, and specifically upon Sejanus and the Praetorians.
In CE 17 or 18, Tiberius trims the ranks of the Praetorian guard responsible for the defense of the city, and moves it from encampments outside of the city walls into the city itself, giving Sejanus access to somewhere between six thousand and nine thousand troops.
The Praetorian Guard is an elite unit of the Roman army formed by Augustus in 27 BCE, 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 manage the day-to-day care of the city, such as general security and civil administration.
Furthermore, their presence serves as a constant reminder to the people and the Senate of the substantial armed force that serves as the basis for the imperial power.
Augustus had been careful, however, to uphold the republican veneer of this regime, and had 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.
It is likely that Lucius Seius Strabo had came to the attention of Augustus through his connection with Maecenas.
Sometime after 2 BCE, Strabo had been 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 Augustus in 14.
Little is known about the life that Strabo’s son, Lucius Aelius Sejanus, had led prior to this date, but according to Tacitus, he accompanied Gaius Caesar, adopted grandson of Augustus, during his campaigns in Armenia in 1 BCE.
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.
When Strabo was assigned to the governorship of Egypt in 15, Sejanus became the sole commander of the Praetorians and had instigated reforms that have helped shape the guard into a powerful tool of the principate.
Sejanus had served the imperial family for almost twenty years when he became Praetorian Prefect in CE 15.
