Polybius is the most trustworthy guide to one of the most momentous periods in Roman history. He also has had an immense impact on historiography. In the eighteenth century he was regarded as a prime source for the history of Federalism, and his theory of the mixed constitution with its system of checks and balances had a profound influence on the framers of the U.S. Constitution. In the nineteenth century Polybius was the model of the critical, scientific, and objective historian. And in the twentieth century his reputation as a historian of the first rank has remained intact. But scholars have viewed Polybius primarily as a Machiavellian pragmatist for whom only political success and power mattered.

Polybius's prose is of a straightforward style that rejects rhetorical embellishment and literary ornamentation, and by comparison with what is known of the historical writings of his contemporaries, he employed rigorous...