Unlike his predecessors, Gaius came to the throne with no military experience. One of the key functions of the emperor and the imperial family under Augustus had been to command the army. Under Tiberius, however, after the invasions of Germany led by Germanicus and Germanicus’ sojourn in the East, military policy had been general pacific and non-expansionist. This must have weakened the association of emperor and military leadership.
Gaius had the advantage of his father’s prestige and reputation. Macro delivered the praetorian guard and the soldiers and generals in the provinces appear to have been satisfied with his accession. There was no repeat of the mutinies that followed on from the death of Augustus.
Nevertheless, it was still early in his short reign (AD 39) that he left Rome for Gaul. Gaul was likely not a random choice. There were on-going diplomatic issues with Parthia, which could probably have been escalated into war, but a war in the East would have taken Gaius far from Rome for an extended period. Gaul was closer and Germany had been the scene of his father’s triumphs. He could expect a warm welcome from his father’s legionaries.
The campaign is represented by the sources as another instance of Caligula’s capricious decision making. There may have been a little more to it. There some issues in Britain and a certain Adminius, son of Cunobellinus, had defected to the Romans. This might have encouraged an intervention. Britain was on the Roman map as unfinished business since Caesar’s invasion of 55 and 54 BC, and Augustus had been tempted to invade. It would be a suitable and glorious prospect for the young emperor. Claudius was to invade early in his reign, perhaps for similar reasons.
But the campaign came to nothing. Famously, Gaius had the soldiers collect sea shells. It is conceivable that on arriving at the Channel, Gaius came to understand the logistical difficulties the invasion faced, and perhaps the speed of his march from Rome meant that the naval support was not ready (Suetonius, Gaius 44; 46; Dio, 59.21).
The other possible targets were the Germans. The nature of the campaign is not clear in the sources. There does seem to have been an organised advance across the Rhine, but on finding that Germans had withdrawn, the troops returned. There were some ‘exercises’ in which Gaius ‘raided’ across the frontier, possibly in pursuit of his own troops, but nothing serious appears to have been attempted.
What is notable (and rarely commented upon) is the number of troops assembled for the campaign. Dio (59.22) gives two figures of 200,000 and 250,000 troops. Gaius appears to have recruited heavily in preparation for the campaign and summoned support from other provinces. This was, even in Roman terms, a huge army to put in the field and Gaius must have stripped many of the Western provinces of their troops to assemble so many. In itself, this suggests that the campaign in Germany was no spur of the moment decision, but one which required considerable logistical support and preparation.
It is perhap unsuprising that the Germans were aware of an approaching army of such a size, believed that they were facing a major campaign the like of which they had not seen since Germanicus’ invasions, and fled as far from the frontier as they were able. Gaius was left with no one to conquer. It is also unsurprising that to feed and supply that army, Gaul was ransacked for supplies and money.
At some point, Gaius suspected conspiracies against him. This may also have undermined his confidence. He was saluted by the troops and he made his way home. It was on his return that his relations with the senate became. Uncertain how to celebrate the non-events in Gaul, Britain, and Germany, whatever they did was wrong (Suetonius, Gaius 49).
The only other theatre of military activity was Africa. Very little is know about the campaign (Dio, 59.20.6), but the conflict was sufficiently serious that it continued into the reign of Claudius, who inherited the honours of the Roman victory from his nephew (Dio, 60.8).
Perhaps Gaius’ military activity is more of failure than a farce. Like his predecessor, he had no able general in the family on whom he could rely. One suspects that any such individual would not have survived long anyhow. His time in Gaul may have been ill-conceived, but his political uncertainties were such that he was potentially distracted and unable to give the time necessary for long campaigns across the Rhine or a distant and perhaps worthless island.