Polybius (c. 200–120 bce) is the major extant Greek historian of the middle Hellenistic period. He was a citizen of Megalopolis, literally the ‘Great City’, which had been created in the early 360s, out of some forty pre-existing communities, as both a consequence and a perpetuation of Sparta's humiliation at the hands of Thebes. He spent a good deal of his adult life in futile pursuit of his city's independence from the federal Achaean League, until, by a stroke of irony, he was forcibly removed to Rome as a hostage precisely for the good behaviour of the Achaeans – who had had the temerity to try to escape from under Rome's ever-lengthening and ever-strengthening grip on the Greek peninsula. Rome's victory in the Achaean War, following on a generation after its victory at Pydna in 168 over the last of the Antigonids, meant that from 146 bce mainland Greece south of Macedonia was a Roman protectorate, a province in everything but name. (The name and formal status were imposed in 27 bce, under the new Roman emperor Gaius Julius Octavianus Caesar, known to us by his adopted surname of Augustus – in Greek ‘Sebastos’, ‘the Revered One’.)

Polybius, who in effect ‘crossed the floor of the House’ (that is, went over to the Roman side) during his loose captivity, sealed his conversion by writing a pro-Roman Greek history in forty books (most of which do not survive).