The ancient sources for the reign of the emperor Caligula, consisting primarily of Suetonius and Cassius Dio - the relevant portions of Tacitus' Annals are lost - are so superficial and biased that it is hard enough to put together a reliable reconstruction of the basic chronology, much less separate fact from fantasy. Under the circumstances, in his Caligula: The Corruption of Power, Anthony A. Barrett does a remarkable job sifting through the available literary and archeological evidence to come up with a coherent narrative that portrays the emperor as more a self-absorbed and arrogant autocrat than the psychotic monster of popular imagination.
A case in point is Suetonius' famous - and virtually incomprehensible - tale of Caligula's abortive "conquest" of Britain. In a farcical scene, Suetonius has Caligula march the legions to the shores of the Ocean - and pick up sea shells on the beach:
Finally, as if he intended to bring the war to an end, he drew up a line of battle on the shore of the Ocean, arranging his ballistas and other artillery; and when no one knew or could imagine what he was going to do, he suddenly bade them gather shells and fill their helmets and the folds of their gowns, calling them "spoils from the Ocean, due to the Capitol and Palatine." As a monument of his victory he erected a lofty tower, from which lights were to shine at night to guide the course of ships, as from the Pharos. Then promising the soldiers a gratuity of a hundred denarii each, as if he had shown unprecedented liberality, he said, "Go your way happy; go your way rich."
Then turning his attention to his triumph, in addition to a few captives and deserters from the barbarians he chose all the tallest of the Gauls, and as he expressed it, those who were "worthy of a triumph," as well as some of the chiefs. These he reserved for his parade, compelling them not only to dye their hair red and to let it grow long, but also to learn the language of the Germans and assume barbarian names. He also had the triremes in which he had entered the Ocean carried overland to Rome for the greater part of the way. He wrote besides to his financial agents to prepare for a triumph at the smallest possible cost, but on a grander scale than had ever before been known, since the goods of all were at their disposal.
Piecing together the available evidence, Barrett cogently argues that the scene was the result of miscalculation, not evidence of insanity.
Until the fall of AD 39, the troops on the Upper and Lower Rhine were commanded by Gnaeus Cornelius Lentulus Gaetulicus and Lucius Apronius, respectively. Both commanders had had their problems containing local threats, and the evidence indicates that Gaetulicus in particular had been lax in the discipline and training of his troops. In late October of that year, Gaetulicus was executed on suspicion of treason and replaced by Servius Sulpicius Galba, the future emperor. It is probably that Publius Gabinius Secundus was appointed to replace Apronius in Lower Germany at about the same time. The two new legates, both of whom were excellent choices, set about to reimpose discipline and launched preemptive raids across the Rhine to pacify local tribes. This would be necessary as a precondition for withdrawing troops from the area in order to use them in Caligula's planned campaign against Britain.
Meanwhile, the overeager and inexperienced emperor, rather than wait to insure that an invasion of Britain would be possible in the Spring of 40, was on his way to the German front, probably arriving at the end of October or early November, shortly after the execution of Gaetulicus and confirmation that the situation was secure. There, Caligula apparently accompanied the troops on a stage-managed raid across the Rhine that encountered more difficulties than anticipated.
Those difficulties reflected the core problem. Until the troops were sufficiently trained, and until the area east of the Rhine was thoroughly pacified, an invasion of Britain courted disaster. The fact of the matter is that these preconditions were not going to be in place by the spring AD 40, as Caligula had apparently assumed. The emperor was caught in an embarrassing dilemma. "Caligula thus faced the prospect of an enormous loss of face if he should be obliged to return [to Rome] with expectations unfulfilled."
At this point, Barrett argues, luck may have intervened, pointing to another confused and confusing passage from Suetonius:
All that [Caligula] accomplished was to receive the surrender of Adminius, son of Cynobellinus king of the Britons, who had been banished by his father and had deserted to the Romans with a small force; yet as if the entire island had submitted to him, he sent a grandiloquent letter to Rome, commanding the couriers who carried it to ride in their post-chaise all the way to the Forum and the House, and not to deliver it to anyone except the consuls, in the temple of Mars the Avenger, before a full meeting of the senate.
Without getting enmeshed in the obscure intricacies of Iron Age British politics, suffice it to say that a member of the royal house of a British tribe was prepared to defect and submit to Rome. Caligula may have used this "heaven-sent opportunity" to create a seaside spectacle that demonstrated that no armed invasion was necessary because Britain had already submitted:
The "surrender" would have been staged with due military pomp - a surrender, after all, can properly be made only to superior military forces. . . . It might have been arranged that the submission would take place at sea, with Caligula sailing out to receive Adminius in the trireme mentioned in Dio, and it is noteworthy that Caligula arranged the trireme should be taken back to Rome to be a part of his triumph. A victory over Britain in such circumstances would represent in the Roman mind a victory over Oceanus. Thus a symbolic collection of shells, the "booty" of Oceanus, to be part of the offering of spoils for the Capitoline, would not be out of order.
Having made the best he could out of a bad situation, Caligula left the north no later than March AD 40. He was back in the vicinity of Rome by the end of May.