As a young man, the historian Polybius was an active politician in the Achaean Confederacy of the second century B.C., and later, during his detention at Rome, became a close friend of some leading Roman families. His History is our most important source for the momentous half-century during which the Romans weathered the war with Hannibal and became masters of the Mediterranean world. F. W. Walbank describes the historical traditions within which...