Of the forty books which made up the history of Polybius, the first five alone have come down to us in a complete form; of the rest we have only more or less copious fragments. But the general plan and scope of the work are explained by Polybius himself. His intention was to make plain how and why it was that “all the known regions of the civilized world had fallen under the sway of Rome”. This empire of Rome, unprecedented in its extent and still more so in the rapidity with which it had been acquired, was the standing wonder of the age, and “who,” he exclaims, “is so poor-spirited or indolent as not to wish to know by what means, and thanks to what sort of constitution, the Romans subdued the world in something less than fifty-three years?” These fifty-three years are those between 220 (the point at which the work of Aratus ended) and 168 B.C., and extend therefore from the outbreak of the Hannibalic War to the defeat of Perseus at Pydna. To this period then the main portion of his history is devoted from the third to the thirtieth book inclusive. But for clearness’ sake he prefixes in Books 1 and 2 such a preliminary sketch of the earlier history of Rome, of the First Punic War, and of the contemporary events in Greece and Asia, as will enable his readers more fully to understand what follows. This seems to have been his original plan, but at the opening of Book 3, written apparently after 146, he explains that he thought it desirable to add some account of the manner in which the Romans exercised the power they had won, of their temperament and policy and of the final catastrophe which destroyed Carthage and for ever broke up the Achaean League. To this appendix, giving the history from 168–146, the last ten books are devoted. - Summary by Encyclopædia Britannica (1911)