Dio Cassius 40, 19-22.2, translated by Mary Beard and Neil Wright

While these events were taking place in Rome, at the very same time Aulus Plautius, a most respected senator, led an expedition to Britain. A certain Bericus who had been exiled from that island as a result of a political dispute had persuaded Claudius to send a force there, and so Plautius assumed command. However, he had trouble taking the sol­diers out of Gaul, as they were uneasy about serving beyond the boundaries of the inhabited world. They could not be induced to obey him until Narcissus, an ex-slave, who had been sent out by Claudius, tried to get up onto the rostrum of Plautius and make an address. They then became much more angry at Narcissus and did not allow him to speak; instead they shouted out the sarcastic comment ‘O Saturnalia’ and immediately followed Plautius quite willingly. Because of these delays they made their departure somewhat late. The troops were divided into three groups so that they should not be prevented from making a landing, as they might have been, had they crossed in a single body. Early in the voyage they were disheartened when the wind turned against them, but they were later encouraged when a light rising from the east ran across the sky towards the west, the direction in which they were bound. They landed on the island with no opposition. For the Britons, from information they had received, had not expect­ed the Romans to come and so had not assembled in readiness. In fact, even when they did assemble, they failed to engage the enemy in close combat but retreated to the marshland and the forests, for they hoped to wear out their foe by different means and force them to sail away unsuccessful, as had happened when Julius Caesar came.
Plautius encountered great difficulty in discovering them, but when he did come upon them he defeated first Caratacus then Togodumnus, both sons of Cunobelinus, now dead. When they had fled he brought over by negotiation a part of the Bodunni, previously under the control of the Catevellaunian princes, and then, leaving a garrison, proceeded further. Eventually he came to a river which the natives thought he would be unable to cross without bridges. Consequently they had encamped somewhat carelessly on the bank opposite. Plautius sent over the German auxiliaries who were used to swimming easily across even the fastest flowing rivers without removing their equipment. These attacked the enemy unexpectedly and, instead of shooting at the men, wounded the horses drawing the chariots and threw them into such confusion that their occupants were unable to reach safety. He also sent across Flavius Vespasianus and his brother Sabinus, who was his subordinate. And so they managed to cross the river and kill many of the natives who were taken by surprise. However, the rest did not take flight but, attacking again on the next day, engaged in a struggle with the Romans that was not to be resolved until Gaias Hosidius Geta, after risking his own capture, inflicted such a severe defeat upon them that he was awarded triumphal ornaments, although he had never held the consulship. The Britons then retreated to the river Thames or, more precisely, to the mouth of that river, where it forms a lake at flood tide. This they forded with ease on account of their exact knowledge of the safe and accessible ground in the area. At first the Romans were confused by the ground in their attempt to follow but then the Germans managed to swim across again and other troops crossed on bridge a little further upstream. Once on the other side they attacked the natives from several directions simul­taneously and slaughtered many of them. But, as they followed up the remainder without due care, they became entangled in the trackless marshland and lost many men.
However, at the death of Togodumnus, so far from surrendering, the Britons united all the more strongly against the Romans to avenge him. Worried by this and by the minor reverse they had suffered earlier, Plautius did not advance any further but consolidated his gains with garrisons and sent off for Claudius. He was under orders to do this if serious opposition was encountered, since many and varied supplies (along with elephants) had been prepared to reinforce the expedition.
On the arrival of the message, Claudius handed over to his colleague in the consulship, Lucius Vitellis, control of affairs at home, including command of the troops stationed in Italy. Claudius himself left on campaign; he sailed down to Ostia and from there was con­veyed round to Massilia, from which point, journeying partly on foot and partly on river transport, he arrived at the Atlantic coast. After he had made the crossing to Britain, he joined the army awaiting him in camps by the Thames and assumed overall command. Then, once having crossed the river, he engaged the natives who had gathered together to repel his attack and defeated them in a pitched battle. He also took Camulodunum, the royal seat of Cunobelinus. As a result of this victory he brought over many of the Britons, some by negotiation, others by force, and was therefore hail-Imperator many times, contrary to custom. He proceeded to disarm these natives and entrusted them to Plautius whom he instructed to bring the remaining territory under Roman control. Claudius himself returned to Rome, sending his sons-in-law, Magnus and Silanus, ahead of him with news of the victory. When the Senate learnt of his success, they gave him the honorific title ‘Britannicus’ and allowed him to hold a triumph. They also voted him a yearly festival and two triumphal arches, one in Rome and one in Gaul, since it was from that country that he had crossed to Britain. The same title, ‘Britannicus’, was conferred on his son, a title which by its habitual use came to be his regular name. Finally Messalina was granted the privilege of occupying a front seat in the theatre (as Livia had been before) and permission to use a carriage within the city of Rome.