Figure 6. Coin depicting Caligula. RIC 37 (Gaius).
What actually occurred? Tacitus notes briefly that the emperor Gaius was murdered in “secret treachery” (occultae insidiae), in contrast to Julius Caesar, who was killed by a conspiracy of senators. All accounts are in agreement that the actual murder was committed by two tribunes of the Praetorian Guard, Cassius Chaerea and Cornelius Sabinus, with the assistance of a number of centurions, and that Callistus and the Praetorian prefects were informed about the plan in advance. Flavius Josephus, who provides the most detailed description, on the basis of a nearly contemporary senatorial history, identifies the architects of the conspiracy as one Aemilius Regulus of Cordoba (of whom nothing more is known) and Annius Vinicianus, along with Cassius Chaerea. In Josephus’s own account of the murder itself, however, Regulus plays no role and Vinicianus plays a minor one. Many years later, people in Rome recalled Valerius Asiaticus as the most important leader of the conspirators, but this view is contradicted by Josephus and Dio’s credible accounts of his actions after the murder. Furthermore, Josephus reports that more senators among the emperor’s close associates knew about the planned assassination, but at the same time he writes that the murderers found pretexts to lure them away from the scene before killing the emperor; this obviously argues against the claim that they were aware of the plot.
Josephus provides one further piece of information, although he does not pursue its implications: Callistus, whom he depicts as Caligula’s most powerful and universally feared adviser, not only participated in the conspiracy because he feared for his own life; he even attached himself to Claudius, “secretly going over to his side because he expected that in the event of Gaius’s death the Empire would pass to him and that by laying up beforehand a store of favor and credit for his kindness he would have a basis for preferment and strength similar to what he now enjoyed” (Jos. Ant. 19.66). Callistus also claimed, according to Josephus, that he had been ordered by Caligula to poison Claudius but had found various excuses to delay. Josephus — almost certainly correctly — considers it unlikely that Callistus would have disobeyed such an order, but it does not occur to him that Callistus might nevertheless have claimed to Claudius that the order had been given.
A look at what happened after the murder rounds out the picture. Soldiers of the Praetorian Guard sought out Claudius, escorted him to their barracks, and proclaimed him emperor. Claudius immediately named Rufrius Pollio as new Praetorian prefect, removing from office the two men who had participated in the conspiracy. After the Senate recognized him as emperor the following day, one of his first acts was to get rid of Caligula’s assassins. Chaerea was executed and Sabinus took his own life. Not long afterwards the two most important figures close to Caligula after Callistus, Protogenes and Helicon, were killed as well. And what about Callistus himself?
We know that Callistus remained a central figure under Claudius. In the Annals of Tacitus, which begin again in A.D. 47, as well as in the accounts of Suetonius and Cassius Dio, he is a powerful secretary a libellis, dealing with petitions to the emperor; he and two other freedmen — Narcissus and Pallas, who were responsible for correspondence and finances—“divided the power among themselves” (Dio 61[60].30.6b). Tacitus refers to Callistus’s role in the assassination of Caligula, and characterizes him as someone who “had expert knowledge of the last court… and believed power to be held more securely by cautious than by vigorous counsels” (Tac. Ann. 11.29.2). The three men successfully deposed the empress Messalina in 48 and deliberated afterwards on who should succeed her; Callistus argued for Lollia Paulina, the former wife of Caligula. He was unable to prevent Agrippina, Caligula’s sister, from becoming empress, but he did manage something very rare for people at the center of power in those days, emperors and empresses included: He died a natural death, about ten years after Caligula was murdered.
Back to January 41. No valiant senators eliminated the hated emperor. They had to leave the deed to Caligula’s right-hand man, a former slave with power “no less than a tyrant’s.” The evidence suggests that events ran as follows: Callistus made use of his final option in order to save his own skin. An assassination of Caligula did not suffice by itself. It was necessary besides for a successor to be installed who was thereafter beholden. And the freedman could not have anything to do with the murder itself. No new emperor would have left his predecessor’s killer unpunished, as that would set a dangerous precedent for his own security. The two Praetorian prefects must have accepted this option, although it was even more dangerous for them. Merely knowing of the assassination and doing nothing about it would mean a violation of the oath they had sworn to protect the emperor’s life, and no successor could possibly have retained them in the positions they occupied. It can no longer be established what precise role they played, whether they failed to recognize the danger that a change of ruler represented for their own position, or whether it seemed a lesser threat than what they feared would happen if Caligula remained. They may have counted on a possible outcome that was in fact what actually happened: They were removed from office, but remained unharmed because they had helped to elevate the new emperor to the throne.
Choosing a successor was not a difficult task. The brother of Germanicus, Caligula’s uncle, was the obvious candidate in dynastic terms and appeared harmless as well. The only task remaining was to find an assassin, someone either too dim to realize that he was letting himself in for certain death no matter how the attempt turned out, or else so motivated that he did not care.
The Praetorian tribune Cassius Chaerea is portrayed by Josephus as a kind of Roman hero of the old school, not only willing but eager to liberate Rome from the tyrant even if it cost him his life. Dio writes, however, that he was very old-fashioned and also had a personal motive for the murder. Supposedly Caligula had regularly made him the butt of jokes, teasing him as weak and unmanly; when he asked for the password of the day, Caligula would choose a word such as Priapus or Venus. Josephus provides further background information that depreciates the tribune’s allegedly noble motives. He reports that Caligula had given Chaerea the distasteful task of collecting taxes, including demanding late payments, an assignment that no doubt made him unpopular. When he didn’t perform it to the emperor’s satisfaction, says Josephus, Caligula accused him of cowardice and insufficient manliness and began making jokes at his expense. “Even his fellow tribunes made fun of him; whenever he was to bring them the password from Caesar, they would mention beforehand one of the words that lend themselves to jests.” Lastly the emperor “employed Chaerea in cases of murder and any others that called for torture, because he calculated that Chaerea’s performance would be more cruel, since he would not want to be abused as a weakling” (Jos. Ant. 19.31, 19.34).
In other words, Chaerea was the man who did Caligula’s dirty work. The emperor took advantage of his weaknesses and abused him for his own ends. Now Chaerea came under pressure from another direction. When Pomponius had been charged with crimes and the tribune had tortured Quintilia so brutally that even the emperor was overcome by pity, “these things grievously distressed Chaerea, for he had been, so far as it was in his power, a source of misery to persons who were considered even by Gaius to be deserving of consolation” (Jos. Ant. 19.37). Trying to demonstrate his masculinity when he was torturing a woman, he had proved himself to be more ruthless than the emperor; he could no longer excuse his actions by saying that he was merely carrying out Caligula’s orders. It was this ill-omened situation that made Chaerea dare to bring up the subject of murdering the emperor, in a discussion with the Praetorian prefect Clemens and another tribune named Papinius. In addition to the unselfish motive of striking a blow for freedom Chaerea mentioned further arguments in favor of assassination, including the unsavory role they themselves played in the fate of Caligula’ victims: “We pollute ourselves with shedding their blood and torturing them daily, up to the moment, mark you, when someone as Gaius’s agent will do the same to us. For he will not favor us in his policy on account of these services, but will rather be governed by suspicion, especially when the number of the slain has increased… There we shall be, set up before him as targets, when we ought to be upholding the security and independence of all the people” (Jos. Ant. 19.42–43).