Herodian (late second, first half third century): Greek historian, author of a History of the Roman Empire since the Death of Marcus Aurelius in which he describes the reign of Commodus (180-192), the Year of the Five Emperors (193), the age of the Severan dynasty (211-235), and the Year of the Six Emperors (238).
The translation was made by Edward C. Echols (Herodian of Antioch's History of the Roman Empire, 1961 Berkeley and Los Angeles) and was put online for the first time by Roger Pearse (Tertullian.Org). The version offered on these pages is hyperlinked and contains notes by Jona Lendering.
The British War
[3.14.1]  In the midst of the emperor's distress at the kind of life his sons were leading and their disgraceful obsession with shows, the governor of Britain informed Severus by dispatches that the barbarians there were in revolt and overrunning the country, looting and destroying virtually everything on the island. He told Severus that he needed either a stronger army for the defense of the province or the presence of the emperor himself.
[3.14.2] Severus was delighted with this news: glory-loving by nature, he wished to win victories over the Britons to add to the victories and titles of honor he had won in the East and the West. But he wished even more to take his sons away from Rome so that they might settle down in the soldier's life under military discipline, far from the luxuries and pleasures in Rome. And so, although he was now well advanced in years and crippled with arthritis, Severus announced his expedition to Britain, and in his heart he was more enthusiastic than any youth.
[3.14.3] During the greater part of the journey he was carried in a litter, but he never remained very long in one place and never stopped to rest. He arrived with his sons at the coast sooner than anyone anticipated, outstripping the news of his approach. He crossed the Channelnote[A translation error. The Greek text says Ocean. It is possible that Severus crossed to Britain from Lugdunum at the mouth of the river Rhine.] and landed in Britain; levying soldiers from all these areas, he raised a powerful army and made preparations for the campaign.
[3.14.4] Disconcerted by the emperor's sudden arrival, and realizing that this huge army had been assembled to make war upon them, the Britons sent envoys to Severus to discuss terms of peace, anxious to make amends for their previous errors.
[3.14.5] Seeking to prolong the war so as to avoid a quick return to Rome, and still wishing to gain a victory over the Britons and the title of honor too, Severus dismissed the envoys, refusing their offers, and continued his preparations for the war. He especially saw to it that dikes were provided in the marshy regions so that the soldiers might advance safely by running on these earth causeways and fight on a firm, solid footing.
[3.14.6] Most of the regions of Britain are marshy, since they are flooded continually by the tides of the ocean;note[Of course, this is nonsense, but the ancient Greeks and Romans were fascinated by the strange phenomenon of the tides at the edges of the earth.] the barbarians are accustomed to swimming or wading through these waist-deep marsh pools; since they go about naked, they are unconcerned about muddying their bodies.
[3.14.7] Strangers to clothing, the Britons wear ornaments of iron at their waists and throats; considering iron a symbol of wealth, they value this metal as other barbarians value gold. They tattoo their bodies with colored designs and drawings of all kinds of animals; for this reason they do not wear clothes, which would conceal the decorations on their bodies.
[3.14.8] Extremely savage and warlike, they are armed only with a spear and a narrow shield, plus a sword that hangs suspended by a belt from their otherwise naked bodies. They do not use breastplates or helmets, considering them encumbrances in crossing the marshes. For all these reasons, Severus prepared whatever he thought would be of advantage to the Roman army and whatever would harass the barbarians and hamper their attacks.
[3.14.9] When it seemed to him that all was in readiness for the campaign, Severus left the younger of his two sons, Geta, in the section of the province under Roman control; he instructed him to administer justice and attend to imperial affairs, leaving with him as advisers his more elderly friends. Then, accompanied by Caracalla, the emperor marched out against the barbarians.
[3.14.10] After the troops had crossed the rivers and the earthworks which marked the boundary of the Roman empire in this region,note[The walls of Hadrian and Antoninus Pius.] frequent battles and skirmishes occurred, and in these the Romans were victorious. But it was easy for the Britons to slip away; putting their knowledge of the surrounding area to good use, they disappeared in the woods and marshes. The Romans' unfamiliarity with the terrain prolonged the war.