Tiberius’ first prefect of the guard was L. Seius Strabo. His support was crucial support at the accession and his loyalty was rewarded with a posting as Prefect of Egypt. Although an equestrian, Seius Strabo was connected to several prominent senatorial families. An inscription (ILS 8996) records his building of a bath house together with his mother Terentia, and his wife, Cosconia Gallitta, who may have been the sister of the consul of 10 BC. Terentia may have been from the very prominent Terentii family who were very close to Augustus in the 20s BC. An earlier wife was probably the sister of Junius Blaesus, who had been governor during the mutinies on the Danube. He may also have married an Aelia, again from a very prominent family, who were later to adopt his son.

The son was born Seius Strabo, but on adoption changed his names to L. Aelius Sejanus. The only reason for adoption in the high Roman elite was so that the the adopting man had an heir to his name and his status. The adoption certainly made Sejanus very rich and he could have used his connections to embark on a senatorial career, but he chose or it was decided that he would follow his father to become firstly joint prefect (with his father) and then sole prefect of the praetorian guard.

Sejanus  was sent with the young Drusus to quell the mutiny on the Danube (Tacitus, Annales, 1.24). He appears briefly at the trial of Piso. In AD  20 he betrothed his daughter to Claudius’ son (Tacitus, Annales 3.29), bringing his family into the imperial household. In 21, his influence was behind the appointment of Quintus Junius Blaesus to lead campaigns in Africa (Annales 3.35) in preference to Aemilius Lepidus. He appears tangentially at the trial of Junius Silanus, as the sponsor of one of the prosecuting senators. He had been among the friends of Gaius Caesar (Augustus’ grandson) (Tacitus, Annales, 4.1), but had become an associate of Tiberius. By AD 22, Blaesus was back in Rome. he had defeated the African king, Tacfarinas. Tiberius honoured him by allowing him to be declared ‘imperator’ (general) by his troops:  he was the last non-emperor to be so called (Annales 3.73-74). In the same year, a fire broke out in Rome and badly damaged the Theatre of Pompey. Sejanus himself was honoured by a bronze statue in the theatre on account of his fire fighting effort (Annales 3.72). 

By AD 23, Sejanus was a power in the city. He was well-connected by birth, but became better connected by politics. His uncle was one of the more powerful and prestigious military men. By his support, men rose. By his enmity, men fell.

Sometime early in the reign, Tiberius allowed him to concentrate the praetorian guard, who had previously been scattered throughout Italy, into a single camp in the city. This gave him a force of perhaps as many as 8000 soldiers in a city which normally had almost no garrison or police (Annales 4.2; Dio, 57.19).

The exact chronology is not clear, but it is evident that Sejanus was emerging as a primary representative of imperial power. Augustus and Tiberius had had secondary figures to support them almost throughout their periods of rule: Agrippa, Tiberius and Drusus, Tiberius alone, Gaius and Lucius Caesar, Tiberius and Germanicus had all performed this role for Augustus. Tiberius had relied on Germanicus and Drusus, and now Sejanus. But Sejanus was not (yet) a family member and one might expect that his rise was viewed with some hostility by Drusus.

But the senators responded with honours. Statues were granted and the Roman elite began to pay him court. Speeches were delivered in his honour (Dio, 57.21). Tiberius appears to have referred to him as ‘socius laborum’ (ally in our labours’) and allowed his image to be sent round the empire to towns and to the army, where it was displayed in the camps (Annales 4.2). This is was the mark of someone being groomed for an imperial position.

‘Socius laborum’ is an odd phrase. It is almost as if Tiberius was avoiding saying something else. That something else was supposedly referenced by Drusus, who complained of Sejanus ‘and how close was he to be declared a colleague?’ There was a crucial movement from being a helper to being an ally to being a colleague (Annales 4.7). ‘Colleague’ was a constitutional word, suggesting a partner in power. Drusus complained that he and his family were being usurped and with the concentration of military power, honours, and statues, friends and influence, and his familial connection to the imperial family, Sejanus was on the verge of imperial power. With relations between the two so poor, they seemingly fell, on one occasion,to brawling (Annales 4.3).

In AD 23, Drusus died. There was no doubt who was the beneficiary and was widely thought that he was poisoned. It was not until the fall of Sejanus that evidence emerged that seemingly confirmed those suspicions.

Sejanus was dominant and that dominance brought him into conflict with the remaining figures of the imperial family, notably Agrippina and her two eldest sons, Nero Caesar and Drusus.