Historical references to the Celts begin in the fifth century BCE. Herodotus and Hecataeus of Miletus are the forerunners of a long series of Greek and Latin writers whose reports and comments, both well- and ill-informed, reflect the changing fortunes of the Celtic peoples during the pre-Christian era and their impact on the Greco-Roman world. Herodotus and Hecataeus confirm that by about 500 BCE the Celts were already widely dispersed over central and western Europe, including perhaps Gaul and the Iberian Peninsula, and evidence from the fifth century testifies to further territorial expansion. About 400 BCE this process quickened as tribal bands invaded northern Italy and established settlements that, in due course, became the Roman province of Gallia Cisalpina. Some Celtic bands raided farther south, as far as Rome and Apulia and even Sicily, and around 387 they captured and sacked the city of Rome, an event of traumatic...