Niccolò Machiavelli only mentions Lucius Aelius Sejanus (aka Sejanus; 20 BCE – CE 31) once in his works: in a single paragraph found in his Discourses, Book III, Chapter VI—Of Conspiracies. Even then, Machiavelli only mentions him in passing in a very short list of conspirators (trans. Marriott, emphasis added):
We see, however, that the great majority of conspirators have been persons of position and the familiars of their prince, and that their plots have been as often the consequence of excessive indulgence as of excessive injury; as when Perennius conspired against Commodus, Plautianus against Severus, and Sejanus against Tiberius; all of whom had been raised by their masters to such wealth, honours, and dignities, that nothing seemed wanting to their authority save the imperial name. That they might not lack this also, they fell to conspiring against their prince; but in every instance their conspiracies had the end which their ingratitude deserved.
This oversight surprises me because the life and actions of Sejanus seem a rich topic for Machiavelli to have elaborated on, especially in his chapter on conspiracies. Yet Machiavelli has no more to say about the second-in-command under the Roman emperor Tiberius who came within throwing distance of overthrowing the emperor. Sejanus was a master conspirator until, of course, his final, failed gamble. But he came so very close to winning.
Not even in The Prince does he get a mention as a warning to leaders who put too much trust and power in their subordinates, as he does in Chapter 22. But why not? Sejanus seems tailor-made as an icon of misplaced confidence.
(BBC radio serialization of Robert Graves’ book, Episode 3 on Sejanus can be heard here.)
Sejanus was not in line for the succession, not even in the Julian family, but he wanted the crown, so he plotted to gain it by other means. Sejanus was an able, competent, and effective administrator. He had the full confidence of Tiberius and, at least at first, appeared entirely subservient to the emperor. But he had long-term plans well above his station.
First, he eliminated the sons of Tiberius, Drusus and Germanicus, making both deaths seem like either an accident or the work of someone else. Tiberius was shorn of his heirs in quick succession.
Sejanus continually undermined the position of Drusus’s mother, Vipsania Agrippina, and tried to marry Drusus’s widow, Livilla to get into the family, but the emperor refused permission. So instead, Sejanus persuaded the 64-year-old emperor to retire to Capri, leaving the empire in Sejanus’ hands while Tiberius engaged in dissolute pursuits. Tiberius didn’t even return to Rome for his mother’s funeral. Sejanus then had Agrippina and her son, Nero, exiled and out of the way.
As head of the Praetorian Guard, Sejanus had a personal army at his disposal to enforce his decisions. He took over more and more of the running of the government.
Tiberius, the emperor, was happiest as a general fighting his battles, and unhappy when having to deal with the business of government. He had become increasingly embittered over having to manage it and was happy to spend his last days in Capri paying little to no attention to the empire. He let Sejanus take more and more authority onto his shoulders, and Sejanus expanded his power and base as he held the reins, making himself even more powerful than the Roman senate and in line to take over the highest seat.
Sejanus still had the emperor’s nephews (the children of Drusus) to deal with: Drusus, Nero and Gaius. Tiberius exiled Agrippina (accused of adultery) and her son Nero; Nero committed suicide and Agrippina starved herself to death. Sejanus also got Tiberius to condemn and imprison Drusus, who then committed suicide. But Tiberius took with him his 17-year-old nephew, Gaius (Gaius Julius Caesar Augustus Germanicus, aka Caligula), making him inaccessible to Sejanus’s machinations.
Seeing himself thwarted from marrying Livilla, Sejanus wooed Gaius’ sister, Livia Julia and finally got the emperor’s permission to marry her. Together they plotted to eliminate Tiberius. But Tiberius was being warned by his sister-in-law Antonia about Sejanus’s designs and began a counter-plot to save himself. The emperor replaced Sejanus as the head of the Praetorian Guard (with Naevius Sutorius Macro, who would later be implicated in murdering Tiberius to get Gaius on the throne). Then when Sejanus thought he was being awarded a new and higher position, Tiberius had his administrator arrested and executed. The senate rejoiced.
Macro would prove to be the next Sejanus to the new emperor, Caligula, in another story of betrayal and manipulation, but Machiavelli never even mentioned Macro (in fact, he only mentions Caligula twice in The Discourses and not at all in The Prince).