Final answer:
The Romans initially conquered the Celts after expanding beyond Italy. Britain was incorporated into the empire in 43 CE, following Claudius' invasion. Later, Anglo-Saxons replaced Roman rule after the Romans' withdrawal from Britain around 400 CE.
Step-by-step explanation:
The Celts, a tribal society spread across Western Europe, were first conquered and then assimilated by the Romans. This occurred after the Romans expanded beyond Italy and saw the Celts as barbarians capable of adopting Roman civilization. Following Julius Caesar's invasion in 55 BCE, there were increasing encounters between the Celtic and Roman cultures. It was Emperor Claudius' invasion in 43 CE that led to southern Britain's incorporation into the Roman Empire. This conquest involved the establishment of Roman roads and forts, such as Hadrian's Wall, and the imposition of Roman cultural practices within the region.
The Anglo-Saxons, from present-day northern Germany and Denmark, invaded and settled Britain after the Romans retreated around 400 CE. The Germanic invaders pushed out the remaining Romans and established control, leading to a period where Roman culture nearly vanished from England, and the history of Anglo-Saxon England began.