DIO CASSIUS (more correctly Cassius Dio), Cocceianus (c. A.D. 150–235), Roman historian, was born at Nicaea in Bithynia. His father was Cassius Apronianus, governor of Dalmatia and Cilicia under Marcus Aurelius, and on his mother’s side he was the grandson of Dio Chrysostom, who had assumed the surname of Cocceianus in honour of his patron the emperor Cocceius Nerva. After his father’s death, Dio Cassius left Cilicia for Rome (180) and became a member of the senate. During the reign of Commodus, Dio practised as an advocate at the Roman bar, and held the offices of aedile and quaestor. He was raised to the praetorship by Pertinax (193), but did not assume office till the reign of Septimius Severus, with whom he was for a long time on the most intimate footing. By Macrinus he was entrusted with the administration of Pergamum and Smyrna; and on his return to Rome he was raised to the consulship about 220. After this he obtained the proconsulship of Africa, and again on his return was sent as legate successively to Dalmatia and Pannonia. He was raised a second time to the consulship by Alexander Severus, in 229; but on the plea of ill health soon afterwards retired to Nicaea, where he died. Before writing his history of Rome (Ῥωμαικά or Ῥωμαικὴ Ἱστορία), Dio Cassius had dedicated to the emperor Severus an account of various dreams and prodigies which had presaged his elevation to the throne (perhaps the Ἐνόδια attributed to Dio by Suidas), and had also written a biography of his fellow-countryman Arrian. The history of Rome, which consisted of eighty books,—and, after the example of Livy, was divided into decades,—began with the landing of Aeneas in Italy, and was continued as far as the reign of Alexander Severus (222–235). Of this great work we possess books 36-60, containing the history of events from 68 B.C.–A.D. 47; books 36 and 55-60 are imperfect. We also have part of 35 and 36-80 in the epitome of John Xiphilinus, an 11th-century Byzantine monk. For the earlier period the loss of Dio’s work is partly supplied by the history of Zonaras, who followed him closely. Numerous fragments are also contained in the excerpts of Constantine Porphyrogenitus. Dio’s work is a most important authority for the history of the last years of the republic and the early empire. His industry was great and the various important offices he held afforded him ample opportunities for historical investigation. His style, though marred by Latinisms, is clearer than that of his model Thucydides, and his narrative shows the hand of the practised soldier and politician; the language is correct and free from affectation. But he displays a superstitious regard for miracles and prophecies; he has nothing to say against the arbitrary acts of the emperors, which he seems to take as a matter of course; and his work, although far more than a mere compilation, is not remarkable for impartiality, vigour of judgment or critical historical faculty.
The best edition with notes is that of H. S. Reimar (1750–1752), new ed. by F. G. Sturz (1824–1836); text by I. Melber (1890 foll.), with account of previous editions, and U. P. Boissevain (1895–1901); translation by H. B. Foster (Troy, New York, 1905 foll.), with full bibliography; see also W. Christ, Geschichte der griechischen Litteratur (1898), p. 675; E. Schwartz in Pauly-Wissowa’s Realencyclopädie, iii. pt. 2 (1899); C. Wachsmuth, Einleitung in das Studium der alten Geschichte (1895).