Drusus, who had become a consul in the year 15, had served as governor of Illyricum from 17 to 20.
In 21 he was consul again, significantly with his father Tiberius as his colleague, while in 22 he had received tribunicia potestas (tribunician power), a distinction reserved solely for the emperor or his immediate successor.
Drusus had married his paternal cousin Livilla in 4; their daughter Julia was born shortly after.
They had twin sons in 19: Tiberius Gemellus and Tiberius Claudius Caesar Germanicus II Gemellus, the latter of whom dies still an infant in 23.
Germanicus’ death in 19 had made Drusus the new heir; Germanicus' wife Agrippina suspects Tiberius of having killed him to allow Drusus to become his heir, but this is unlikely.
Before the birth of the twins, Livilla may already have been in a relationship with Sejanus, who, in his capacity of Praetorian prefect Sejanus had quickly become a trusted advisor to Tiberius.
By 23, he already exerts a considerable influence over the decisions of the emperor, who referrs to Sejanus as "Socius Laborum" (my partner in my toils).
By this time he has been raised to the rank of praetor, a position which is not normally granted to Romans of the equestrian class.
A statue has been erected in his honor in the Theater of Pompey, and his followers in the Senate are advanced with public offices and governorships.
However, this privileged position causes resentment among the senatorial class and the imperial family, in particular earning him the enmity of Drusus.
The history of Sejanus and Drusus dates to at least 15 CE, in which year a mutiny had broken out among legions posted in Pannonia and Germania.
While his adopted son Germanicus restored order in Germania, Tiberius' biological son Drusus had been 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 had quickly managed to restore the stability in the army and publicly put the chief instigators to death.
The camp had been purged of mutineers by the Praetorians and the legions returned to the winter barracks.
Despite this success, the following years have witnessed a growing animosity between Drusus and Sejanus.
Since the accession of Tiberius, Drusus has 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 is still Sejanus who is the second man in the empire, and he is 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 four 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 had turned his attention toward eliminating Drusus.
By 23, the enmity between the two men has 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" (Tacitus, Annals IV.7).
With Tiberius already in his sixties, there is a real possibility of Drusus succeeding his father in the near future.
Ancient sources (Tacitus, Suetonius, Cassius Dio) agree that, to secure his position, Sejanus secretly plotted against him and succeeded in seducing his wife Livilla.
With her as an accomplice, Drusus is slowly poisoned and dies of seemingly natural causes on September 13, 23.
If Drusus was indeed murdered, then it was done so skillfully that his death gives rise to no suspicion, having as he does a reputation for heavy drinking.
