Cassius Dio, a Bithynian Greek who later became a Roman senator and consul, is known primarily for his partially extant Roman History (early third century A.D.). This work, eighty books long, was the last full history of Rome to be written in antiquity; it covers the more than one-thousand-year period from the supposed arrival of Aeneas in Italy in the eleventh century B.C. until Dio's own consulate in A.D. 229. During a public career in which he was remarkably adept at shifting with changing political winds, Dio prospered under a series of emperors from Commodus (who ruled from A.D. 180 to 192) to Severus Alexander (who reigned from A.D. 222 to 235); Dio preserves in his remarkable history firsthand accounts of all of them. In A Study of Cassius Dio (1964) Fergus Millar writes that there exists for Dio "a more valuable record of personal experience than for any...