CASSIUS DIO (or, in Greek style, Dio Cassius) was Greek – born at Nicaea, in the Roman province of Bithynia. He was a Roman senator, and author of Roman History – a history of Rome, written in Greek, from its legendary beginnings up to his own times. It is from casual comments he makes within the work that some details of his life are known.

Dio’s father, Cassius Apronianus, had been a successful senator – achieving a consulship and a number of governorships (Dio had accompanied him on his governorship of Cilicia). Dio himself became a senator under Commodus (180–192). In 193, Pertinax nominated him for the praetorship. Pertinax’s reign, however, lasted only three months, so it was under Septimius Severus (193–211) that Dio served his year in office.

He had the dubious pleasure of spending the winter of 214/5, at Nicomedia in his native Bithynia, in the company of, Severus’ son and successor, Caracalla (211–217). He was put in charge of the cities of Pergamum and Smyrna by Macrinus (217–218) – a position he retained under Elagabalus (218–222). At some stage (c.205 seems the most popular suggestion, though c.222 also has its advocates) Dio had been consul, and, early in the reign of Alexander Severus (222–235), following a period of illness spent in Bithynia, he served as proconsul of Africa. He went on to govern Dalmatia, then Upper Pannonia. In 229 he was consul for the second time, as colleague of Alexander himself. However, Dio had felt it necessary to be a firm disciplinarian with his troops in Pannonia, and, as a result, the Praetorians (the emperor’s bodyguard) were hostile towards him. There was fear that his life might be under threat, so he was obliged to spend his time away from Rome. Soon – he says because he had a foot ailment – Dio asked Alexander to excuse him, and he retired to his native country, where he, presumably, died at some time in the 230s.

Dio tells how he came to produce his Roman History:

After this [the assassination of Commodus] there occurred most violent wars and civil strife. I was inspired to write an account of these struggles by the following incident. I had written and published a little book about the dreams and portents which gave [Septimius] Severus reason to hope for the imperial power; and he, after reading the copy I sent him, wrote me a long and complimentary acknowledgment. This letter I received about nightfall, and soon after fell asleep; and in my dreams the Divine Power commanded me to write history. Thus it was that I came to write the narrative with which I am at this moment concerned. And inasmuch as it won the high approval, not only of others, but, in particular, of Severus himself, I then conceived a desire to compile a record of everything else that concerned the Romans. Therefore, I decided to leave the first treatise no longer as a separate composition, but to incorporate it in this present history, in order that in a single work I might write down and leave behind me a record of everything from the beginning down to the point that shall seem best to Fortune. This goddess gives me strength to continue my history when I become timid and disposed to shrink from it; when I grow weary and would resign the task, she wins me back by sending dreams; she inspires me with fair hopes that future time will permit my history to survive and never dim its lustre; she, it seems, has fallen to my lot as guardian of the course of my life, and therefore I have dedicated myself to her. I spent ten years in collecting all the achievements of the Romans from the beginning down to the death of Severus [i.e. 211], and twelve years more in composing my work. As for subsequent events, they also shall be recorded, down to whatever point it shall be permitted me.
Roman History (Epitome, Xiphilinus) LXXIII, 23

The History originally consisted of eighty books. Today, only the section from Book XXXVI to Book LX (inclusive) remains, more or less, intact. The rest exists as fragments (excerpts), and in abridged versions of some sections – known as the Epitomes – from two Byzantine historians:

Book LX of Cassius Dio’s Roman History contains the only extant narrative of the Claudian invasion of Britain (AD 43).

Roman History translation by Earnest Cary.

Bithynia was in northern Asia Minor – adjacent to the Sea of Marmora, the Bosphorus and the south-western corner of the Black Sea. Nicaea is now İznik, Turkey.
Cilicia was in south-eastern Asia Minor, on the Mediterranean coast.
Nicomedia is now İzmit, Turkey.
Pergamum and Smyrna were in the Roman province of Asia (which was the western part of Asia Minor). Smyrna is now İzmir, on the Aegean coast of Turkey. Pergamum, now Bergama, is about 50 miles north of Smyrna.
The province governed by Dio, called Africa, was a belt of land – broadly, north-eastern Algeria, most of Tunisia and a ribbon of territory along the coast of Libya – on the Mediterranean.
The province of Dalmatia faced Italy across the Adriatic. The name is still in use for a coastal region of Croatia.
Pannonia – north of Dalmatia and bounded on its north and east by the Danube – comprised modern western Hungary, and parts of Austria, Slovenia, Croatia, and Serbia. Trajan (98–117) divided the province into Upper Pannonia, or Pannonia Superior (the north and west), and Lower Pannonia, or Pannonia Inferior (the east and south).
Photius (c.810–c.893) Bibliotheca 71.
Alain M. Gowing ‘Dio’s Name’ Classical Philology Vol. 85, No. 1 (January 1990).
Margaret M, Roxan Roman Military Diplomas 1978 to 1984 (1985) No. 133.