As a result, shortly before the emperor’s twenty-seventh birthday, a crisis began to brew that would eclipse everything that had gone before. Events in the following weeks show that around the middle of the year a new conspiracy formed, which would take on dramatic dimensions.
At the core of the conspiracy were Lepidus, the emperor’s most important senatorial confidant, and Gaetulicus, the commander in upper Germania. Other participants included members of Caligula’s immediate family: his two sisters, Agrippina and Livilla, to whom in the previous two years the emperor had awarded the highest honors. Agrippina had entered into an affair with Lepidus “out of her lust after power,” as Tacitus puts it (Ann. 14.2.2). A large number of senators were also privy to the plot, among them both consuls — the highest magistrates of the Roman polity — who had taken office on 1 July. The conspirators could thus count on military backing in the Empire, on broad support among the aristocracy, on the most important officeholders in Rome, and on some of the emperor’s closest relatives — meaning that they also had a presumptive future emperor and empress handy. Caligula himself had confirmed the man’s suitability for rule by including him in his own plans for the succession, and the woman provided the prestige of the current emperor as a “dowry,” so to speak. Agrippina’s son Nero even offered a prospective successor in the next generation. These were probably the best conditions for a conspiracy in the whole history of the Roman Empire. All that remained to do was to assassinate Caligula.
But things did not go as planned. The sources do not reveal who betrayed the plot to the emperor, and its full scope does not appear to have been clear immediately. Evidently only Gaetulicus and senatorial circles in Rome fell under suspicion at first. Caligula’s response was swift and effective. In the early days of September he removed the two consuls from office and ordered his minions to break their fasces, the bundle of rods that symbolized their office and the power connected with it. One of the consuls committed suicide. Caligula replaced them with Domitius Afer, mentioned above, who was close to the emperor’s freedman Callistus, and Aulus Didius Gallus, a senator from an obscure family who was known to be ambitious. Presumably at the same time, the emperor withdrew from the Senate’s hold the last military unit that had remained formally under its control, the legion in the province of Africa, and replaced its commander. Next Caligula traveled to the town of Mevania in Umbria. He had given no sign that he planned a longer journey, but in a surprise move he pushed on from there as swiftly as possible toward Germania. The pace of the march is said to have been so fast that the Praetorian Guard had to use pack animals to carry its standards, and orders were given to cities and towns along their route to wet the roads, to keep the dust down. Traveling in Caligula’s retinue were Lepidus, Agrippina, and Livilla, who had not yet come under suspicion.
An attempt to reconstruct in detail what happened in the following weeks and months runs into a number of difficulties. For one thing, what is true of ancient reports about Caligula in general applies in even greater measure to the great conspiracy of mid-39 and his march north to Germania: The central facts of the case are reported explicitly and in a reliable way, since they appear in contexts only indirectly related to Caligula himself. Where Caligula’s own actions are concerned, however, ancient historians try to present them as incoherent and senseless, occasionally entangling themselves in blatant contradictions in the process. Cassius Dio, for example, reports that Caligula ordered the consuls’ fasces to be taken from them and broken because they had failed to celebrate his birthday properly — a remark that at least establishes the date securely. Suetonius claims that Caligula’s sudden expedition to Germania grew out of a plan to add to the Germanic bodyguard that served him as it had his predecessors on the throne. Yet in the same breath he writes that for this purpose legions and auxiliary troops had been gathered from all over the Empire, new recruits had been raised, and provisions collected “on an unheard-of scale.” Dio writes that the threat posed by Germanic peoples was merely a pretext; in reality the emperor was in financial difficulties and organized the military campaign in order to plunder wealthy Gaul. Yet he mentions only a few sentences later that the troops assembled for the purpose numbered between 200,000 and 250,000, and that the money raised in Gaul was used mainly to pay for this army. In addition both authors’ accounts of how the conspiracy was put down and how the campaign proceeded in Germania portray Caligula’s behavior as absurd and grotesque — thereby demonstrating above all, once again, that this was not what happened.
Modern scholars have debated the subject at length. While the state of the sources means that some questions cannot be answered with certainty, nevertheless it is possible to trace in outline a course of events that appears plausible. As in other distorted accounts of the emperor, we can deduce a basic framework from the overall pattern of events, the mention of details in parallel sources that have no reason to seem suspect, and, last but not least, information that Suetonius and Dio included even though it contradicted the impression they wished to create, probably because the facts were too well known to be suppressed.
To begin with, Caligula’s abrupt departure for the North clearly accomplished his primary purpose. Gaetulicus was taken by surprise and had no time to prepare his legions for an open uprising against the emperor. He was executed, presumably in Mainz, and replaced by Servius Sulpicius Galba, a capable general who was to become emperor himself briefly a few decades later. The full scope of the conspiracy apparently came to light only at this point, perhaps because Gaetulicus betrayed the others in an attempt to save his own skin. Lepidus, Agrippina, and Livilla were found guilty as accessories to the plot to assassinate the emperor; Lepidus was executed, and the two sisters were banished to the Pontine Islands. Caligula forced Agrippina to take an urn with the ashes of her lover Lepidus back to Rome, carrying it against her body for the whole journey. The emperor divulged documents in their own hands revealing their share in planning the conspiracy. He also distributed money to the soldiers as a reward for their continued loyalty to him, and sent the three swords with which the plotters meant to murder him to Rome, where they were placed in the Temple of Mars Ultor (“Mars the Avenger”) as dedicatory offerings. Last, he informed the Senate in a letter about the assassination he had narrowly escaped and forbade the senators to vote honors for any of his relatives in the future. The dating of these events can be reconstructed from a fragmentary inscription of the priestly college known as the Arval Brethren. On 27 October 39 they performed a sacrifice to offer thanks that “the nefarious plans of Gnaeus Lentulus Gaetulicus against Gaius Germanicus were detected.” It can thus be inferred that reports of the general’s disloyalty had reached Rome by then, but the guilt of Lepidus and the emperor’s sisters had not yet been made public.