One the key elements of Tiberius’ reign was his relationship with the senate. The account in Cassius Dio, 57, starts well, but rapidly declines into a litany of prosecutions and deaths though much of book 58. Dio’s account is not always strictly chronological, but in the period before the Germanicus and before his retreat to Capri in AD 26 (Annales 4.57). Dio associates the shift to Capri with a more difficult relationship between Tiberius and his mother and the gradually increasing influence of Sejanus.

A similar pattern can be seen in Suetonius. In Suetonius, Tiberius 26-32, we have Tiberius honouring the senate and refusing honours. But that changes in Tiberius 33,and forms part of a narrative of immorality and cruelty that continues until Tiberius 66. Much of the narrative focuses on his sexual behaviour, which is an important sub-plot to our understanding of Tiberius.

Although Tacitus is similarly inclined to see the retreat to Capri as related to the machinations of Sejanus, his moment of transformation comes earlier, in 22, in the consulship of Sulpicius and Haterius. Although there were treason trials earlier, it with events of AD 22 that there is a significant shift in Tacitus’ account.

In Annales 3.65 and 66-70, Tacitus moves to an extended account of the trial of Gaius Junius Silanus. It is worth looking at the trial in some detail as an example of a pattern that is subsequently repeated. The trial is followed by another trial, that of Ancharius Priscus, which collapses.

These trials are introduced by an extended account of Tiberius granting more influence to the senators by allowing them to receive embassies and make decisions in regard to communities within the imperial state (Annales 3.60-64). We thus have a paradox in which Tiberius appears to be granted more authority to the senate and the relationship between emperor and senate and between senator and senator was worsening.

Tacitus observes this development in Annales 3.65. This chapter

• apologises for the catalogue of depressing trials that will follow.
• asserts that the duty of the historian is moral: to praise the worthy and denounce the scandalous and wicked.
• notes the moral problems of the age (not just of the individuals).
• recounts the sycophantic nature of the senators’ treatment of Tiberius.
• says that whenever Tiberius left the senate, he mutters (in Greek)  ‘men ready for slavery!’

The representation in the text is that slavery emerges from the behaviour of the senators.

There are various historical stages to this emergence:

• Trials before Sejanus
• Sejanus.
• The retreat to Capri.
• The dispute with Agrippina.
• Treason and corruption trials.

But the key elements of the story are a progressive worsening of relations in the senate and between senators and emperor. Given that in the early part of the reign, all authorities are clear that Tiberius tried to give more authority to the senators, we are left with a substantial problem.

• Why did Tiberius’ relations with the senators progressively worsen?

It is worth noting that end of the Augustan era was also a difficult period for senators and emperor. One may wonder whether Tiberius did anything very differently from Augustus. But still it all went wrong.