Was it a good career path to become a prefect of the Praetorian Guard under the Julio-Claudians? This question is debated.
The Praetorian Guard and its origins
Originally, a general in the Republic was protected by a guard called the cohors praetoria, named after the commander's headquarters (praetorium). Some say that the word "praetorian" also derives from the word "praetor", whereas this guard would protect a praetor on campaign. This guard eventually became a personal bodyguard for the faction leaders during the Civil Wars, each of them having one or more cohorts of praetorians.
In 27 BC, Augustus transformed these troops into the core of his own elite bodyguard army at Rome and in Italy, consisting of 9 cohorts of 1,000 (possibly 500) men. According to Dio, there were some 10,000 men deployed in 10 divisions. Augustus actually created a total of 12 cohorts, nine for himself and three (the Urban cohorts) for the Senate, which was the same proportion used in the legions (¼ for the Senate and ¾ for the princeps). The praetorian troops enjoyed superior conditions, such as better pay, shorter length of service and a dressier uniform (even though they did not patrol in dress uniform, to avoid alarming the senators who were not accustomed to troops in Rome or Italy). Augustus actually did not station these troops in Rome proper, but outside. Most of the men in the Guard were of Italian origin.
The main function of this elite army was to be the protection of the princeps. It was hoped that they would deter would-be plotters and conspirators against the princeps. Part of the Guard would also follow the emperor on campaigns, but these would not affect how the generals planned their campaigns. Indeed, to this day, the Praetorians evoke a certain image, as plotters and conspirators themselves and definitely not as competent soldiers. One can think of Nixon's "praetorians" as an example of this association.
In a speech to Augustus, Maecenas proposed several modifications to the running of the state. Amongst others, he recommends that Augustus appoint the two best equestrians to command the praetorian guard and the rest of the soldiers in Italy. He apparently recommends two prefects to ensure that Augustus always have someone to guard him. On the other hand, this position was bound to be very influential, because of the closeness of the prefects to the center of power and of the number of men they commanded, so Augustus surely deemed it wise to use the old republican principle of checks and balances by appointing two prefects. The prefect was not really a second-in-command to the princeps, but by being so close, he would naturally be asked by the emperor informally to accomplish certain duties that were not related to his primary job function, and thus acquired a certain additional influence. Indeed, some prefects, like Sejanus, became overly powerful because of the princeps's dependence on them for the execution of all these duties.
Augustus had also chosen equestrians because he may have thought that they would not be interested in ruling, but also because he was not sure if a senator would accept an unelected post such as one of the new prefectures. There were also not many precedents for a senator in such a function. Augustus also probably gave the equestrians these posts because he thought they would be more loyal to him than the senators. Actually, many prefects had senatorial blood or ties: for instance, L. Seius Strabo's mother was a Terentia. And then again, C. Nymphidius Sabinus was he son of a freedman or freedwoman from the court.
For Augustus, the praetorians were to be a patent demonstration of his armed might, and their loyalty, as well as their discipline, was to be a symbol of the stability of his reign.
The first prefects
In 2 BC, Augustus appointed the first pair of prefects, Quintus Ostorius Scapula and P. Salvius Aper. Very little is known of these two men, except some of the family ties of the Ostorii Scapulae. Apparently, Augustus appreciated this family quite well, and also appointed a Publius Ostorius Scapula to the prefecture of Egypt; he may have been the brother of Quintus. The family was of equestrian rank and of Italian origin and they had ties with the noble Domitii and Sallustii families. A Sallustius Crispus (and Maecenas himself before that) might have even had the duties of a praetorian prefect in an unofficial manner before the appointment of the pair. We do not know how long these prefects were in function, and who the other prefects may have been under Augustus, except for a possible Varius Ligur or Valerius from Liguria, maybe one and the same person. We do have some information on L. Seius Strabo, who was one of the prefects at the end of the reign of Augustus and at the beginning of Tiberius's principate. Seius, and consequently, his son Sejanus, were related to the Cornelii, to the Terentii and to the Junii. Seius kept his post under Tiberius for a few months, was part of the princeps's consilium, and was eventually promoted in AD 15 or 16 to the prefecture of Egypt, probably the top equestrian post of the day.
Sejanus, a would-be successor to Tiberius?
With Sejanus, the son of Seius Strabo, we see the archetype of the haughty and ambitious minister. As we saw earlier, Sejanus was related to several consular families, as well as being himself a prominent equestrian. Also, at least two of his brothers had themselves been consuls. He was born in either 20 or 19 BC, in Volsinii in Etruria. He was named praetorian prefect in AD 14 by Tiberius and worked alongside Seius Strabo for two years, and then was the sole detainer of the post until his death in AD 31. Sejanus was also part of the consilium principis of Tiberius: not many of the men who were part of this council survived the emperor.
Why did Tiberius give Sejanus so much power, with no colleague for checks and balances? For an explanation to this, we might look at Sejanus's youth and at his family ties and political connections. We know that Sejanus probably accompanied or courted Augustus's grandson Gaius before 1 BC. Gaius and his entourage met Tiberius at Samos, where he had come to from Rhodes, and where the future emperor had to assure the princeps juventus that he was not plotting against him. Sejanus might have also served under Tiberius himself in the North and he did serve with Livilla's second husband Drusus in September of AD 14. Some think that by promoting Sejanus to lead the praetorians and by using him in such a position, Tiberius wanted to neutralize a political group of which the prefect was part, based in the ancient Agrippa-Maeceneas rivalry. But Sejanus, even though he was only an equestrian and quite low in this group, actually made a move by concentrating all the Guard at a single barracks, thus dangerously augmenting their concentrated power, and his own at the same time.
In Sejanus's march towards power, we see three possibilities: 1. after Drusus's death brought on by Sejanus and Livilla, Sejanus was to succeed Tiberius and be a caretaker emperor or regent until the sons of Germanicus (or Gemellus)were old enough to be of age (which seems to be the most likely possibility in Tiberius's mind), 2. he was actually aiming for the principate with no strings attached, in which case he was to be locked in a fight to the finish with the Julian line, and 3. he was not considered for any ruling power after the reign of Tiberius, in which case all his acts, including the death of Drusus, seem like a desperate power grab and an actual plot against Tiberius's life would have been in the making.
Let us resume what happened: first, Drusus dies in 23 AD, then Agrippina and Nero are exiled, then Drusus is imprisoned in Rome. Neros is also driven to suicide. Only Gaius seems to have escaped the full force of Sejanus's plotting, as he was in Capri by Tiberius's side and less accessible to the wrath of the prefect. Sejanus was then trying to marry Livilla, to get a foot in the Imperial family's door, and he was also trying to get the son of Livilla declared heir to the throne, probably feeling that he could control him with more ease than the sons of Germanicus.
The main obstacle to Sejanus's ambition seems to have been his birth: as an equestrian, he was not what the Roman aristocracy deemed princeps material. Indeed, he was walking on thin ice where the Senate was concerned, and his power, which was high but was the power of a novus homo, was much resented by the nobility, which is demonstrated in the way Tiberius had him deposed via the conscript fathers. Sejanus also thought he had the backing of the Praetorians, but found out how surprisingly easy it was for them to change their allegiance to a new commander when the emperor commanded it and promised rewards, although their change of heart was not a given in any case. To succeed, Sejanus might have needed stronger support from the lower ranks of the aristocracy, the people and the army. He did get some support from several of the nobility, but not enough in any case to help him overcome the letter of denunciation that Tiberius sent to the Senate via Macro. The only nobles that really supported him were the lesser nobility, because he also supported them when he was dominant. Indeed, he cruelly decimated the ranks of the old nobility over 17 years, by means of prosecutions, executions and enforced suicides, so no support would be forthcoming from the survivors. He also would have had very little support from the populace: it is indeed said the people celebrated his demise with great joy and with the murder of many of his supporters.
Sejanus had tried to have the people consider him as a second Agrippa to counter this impression of lowly birth in the eyes of the people and the Senate, but this did not seem to work very well. He had tried to make his lowly birth an object of admiration because of how far he had come, hence the comparison with Agrippa (and also with the low-born Servius Tullius, to wit a statue of the king - or one belonging to the king - in his own house). This obviously failed, as the minister's popularity compared to Agrippa's was close to nonexistent. Indeed, when the end came, Macro was told to free Drusus Caesar and to show him to the Roman people after Sejanus's imprisonment to counteract any popular movement that the prefect might have had in sympathy to his plight, but this was totally unnecessary, as the people followed him to his imprisonment with "jeers and blows".
My own impression is that Sejanus saw the Julian males attaining the proper age for ruling on their own and was frightened by the possibilities, and that is when the idea of a plot started. Tiberius was old but gave no sign of dying soon, so Sejanus had to start plotting to get rid of any possible threats to his present and future power. Hence, at first, the " poison plot" to kill Drusus, of which Tiberius does not seem to have known that it might have been Sejanus's plotting with Livilla that caused the untimely death of his son, until he received the letter from Sejanus's wife Apicata after her suicide, although this letter can be seen as pure desire of revenge by Apicata and not proof of a plot on the part of Sejanus and Livilla. In this letter, Apicata tells Tiberius that Sejanus organized the murder with the help of Livilla, her doctor Eudemus who was also one of her lovers and a slave called Ligdus who was a lover of Sejanus's: many people of that day believed this story, although it has been now mostly discounted.
Also proving the theory that Sejanus wanted to inherit the principate for himself and not be a caretaker, was the plot against Gaius that had been put into motion by Sejanus and others such as Sextius Paconianus: it might have been this plot that had been the final cause of the prefect's downfall. Tiberius had most likely decided that he was going to pass the power to the Julian line and Gaius had to be protected: if he knew that Sejanus had been trying to get rid of the youngest Julian (and he most certainly knew that Sejanus had plotted against Agrippina, Nero and Drusus beforehand), he would have put the wheels in motion for the minister's arrest and quick demise. Sejanus, however, did not seem to have been actively plotting against Tiberius's life itself. Indeed, even when he knew that all did not quite seem right in his relations with Tiberius, he did not attempt a coup. But the plot against Gaius was a definite possibility. Gaius himself might have been painfully aware of this plot and moved successfully against Sejanus by being the originator of Antonia's famous letter to Tiberius: indeed, Gaius did stay with Antonia before being called to Capri by Tiberius. And knowing the cunning of Gaius, it must have been likely that he did attempt some kind of counterplot against the minister, and succeeded. And it is not proved that Antonia herself wrote the letter, as it is only mentioned directly in Josephus and in passing in Dio. Gaius, indeed, had access to Tiberius (and probably some informers to help keep him informed on Sejanus and his faction) on Capri, although Sejanus himself spent some time on the island in 30, so Gaius would have had to be very careful at that time.
Here we will discuss a bit of Sejanus's psychological makeup. One of the things that did not help Sejanus win allies to his side were his haughtiness: he scared his clients so much that they do not want to be the last to appear before him every day or to not be seen by him at all. Sejanus, according to the knight Titius Sabinus, also had "cruelty, pride and ambition", so his various bad sides were known to the Roman people. He was seen as calculating, spiteful and "evil-minded", indeed, his "good will could only be attained by a crime", when one wanted his support. He was also seen as arrogant, and all the crowds that flocked to him looking for a word or support from the great man must have made him even prouder. However, he must have been charming and convincing to women, because of the way he involved Livilla and Aemilia Lepida in his schemes. And apparently, the women he chose were not only powerful, but beautiful as well, so he must have been fairly attractive himself, even though he was bald. And he must have been persuasive to induce Livilla to abandon her husband and marry him. Also, Velleius Paterculus, who should have obviously been biased by his having to live under the shadow of the minister himself or being one of Sejanus's followers, describes him as loyal and a hard worker, and with a "well-knit body to match the energy of his mind". He was also "stern, but yet gay; cheerful, but yet strict; busy, yet always seeming to be at leisure." Velleius also tells us that Sejanus shunned honors but received plenty, and was humble, calm, but alert.  And he was probably not lacking in courage as well, as can be seen in the famous cave episode: Sejanus had the perfect occasion to prove his loyalty to Tiberius by saving his life when a cavern in a villa called "Speluncae" they were dining in suffered a rock slide, and several people were killed. Sejanus had protected Tiberius with his own body. This episode might also be seen as a move towards added power for Sejanus as well and it is likely that several people were soon to think that this was a calculated act by the minister, which it probably was because Sejanus needed Tiberius alive to further his ambitions, but Tiberius himself may have not really noticed this self-promoting act by the prefect, because he was grateful to still be alive.
An important step in Sejanus's scheme to advance to the principate was his joining in the imperial family by marriage. This he hoped to accomplish in two ways: the first was his own marriage to ...
The Praetorian Prefects under the Julio-Claudians are considered. Extensive research is included.