While the Hellenistic world was flourishing in Greece and the Middle East, and Rome was beginning its long climb from obscurity to power, most of Western Europe was dominated by the Celts. The Celts provide background context to the rise of Rome, since Roman expansion would eventually spell the end of Celtic independence in most of Europe.
Much less is known about the Celts than about the contemporaneous cultures of the Mediterranean because the Celts did not leave a written record. The Celts were not a unified empire of any kind; they were a tribal people who shared a common culture and set of beliefs, along with certain technologies having to do with metal-working and agriculture.
The Celts were a warrior society which seemed to have practiced a variation of what would later be known as feudal law, in which every offense demanded retribution in the former of either violence or “man gold”: the payment needed to atone for a crime and thereby prevent the escalation of violence. The Celts were in contact with the people of the Mediterranean world from as early as 800 BCE, mostly through trade. They lived in fortified towns and were as quick to raid as to trade with their neighbors.
By about 450 BCE the Celts expanded dramatically across Europe. They seem to have become more warlike and expansionist and they adopted a number of technologies already in use further south, including chariot warfare and currency. By 400 BCE groups of Celts began to raid further into “civilized” lands, sacking Rome itself in 387 BCE and pushing into the Hellenistic lands of Macedonia, Greece, and Anatolia. Subsequently, Celtic raiders tended to settle by about 200 BCE, often forming distinct smaller kingdoms within larger lands, such as the region called Galatia in Anatolia, and serving as mercenary warriors for the Hellenistic kingdoms.
Eventually, when the Romans began to expand beyond Italy itself, it was the Celts who were first conquered and then assimilated into the Republic. The Romans regarded Celts as barbarians, but they were thought to be barbarians who were at least capable of assimilating and adopting "true" civilization from the Romans. Centuries later, the descendants of conquered Celts considered themselves fully Roman: speaking Latin as their native language, wearing togas, drinking wine, and serving in the Roman armies.