The Celts present a good example of both assimilation and conflict. Celtic peoples conquered by the Romans often assimilated Roman ways, adapting the Latin language and other aspects of Roman culture. In Roman Gaul and then in Roman Britain, towns were planned in the Roman fashion, with temples, public baths, theaters, and amphitheaters. In the countryside, large manors controlled the surrounding lands. Roman merchants brought Eastern luxury goods and Eastern religions — including Christianity. The Romans suppressed the Celtic chieftains, and a military aristocracy made up of Romans — some of whom intermarried with Celtic families — governed. In the course of the second and third centuries, many Celts became Roman citizens and joined the Roman army. Celtic culture survived only in areas beyond the borders of the empire.

By the fourth century C.E., Gaul and Britain were under pressure from Germanic groups moving westward, and Rome itself was threatened (see Map 7.3). Imperial troops withdrew from Britain in order to defend Rome, and the Picts from Scotland and the Scots from Ireland (both Celtic-speaking peoples) invaded territory held by the Britons. According to the eighth-century historian Bede (BEED), the Briton king Vortigern invited the Saxons from Denmark to help him against his rivals. However, Saxons and other Germanic tribes from the area of modern-day Norway, Sweden, and Denmark turned from assistance to conquest. Their goal was plunder, and at first their invasions led to no permanent settlements. As more Germanic peoples arrived, however, they took over the best lands and eventually conquered most of Britain. Historians have labeled the years 500 to 1066 (the year of the Norman Conquest) the Anglo-Saxon period of English history, after the two largest Germanic groups in England, the Angles and the Saxons.

Anglo-Saxon England was divided along ethnic and political lines. The Germanic kingdoms in the south, east, and center were opposed by the Britons in the west, who wanted to get rid of the invaders. The Anglo-Saxon kingdoms also fought among themselves, causing boundaries to shift constantly. In the ninth century, under pressure from the Viking invasions, King Alfred of Wessex (r. 871–899) created a more unified state with a reorganized army and system of fortresses for defense.