During the last ice age a land bridge connected the British Isles to the European continent and the Thames and Seine rivers were connected. While the land bridge existed, the Iberians, who lived in the area at the time, moved freely between the two land masses. As the ice age ended and the glaciers receded, the English Channel was formed and Britain was permanently separated from the continent. Naturally, some tribes of Iberians remained on the island to inhabit it as permanent residents. The First Invasion: The Celts. About 700 B.C. the Celts invaded the island. This is the first known successful invasion of the island.
The invasion, which occurred over a period of about 400 years, ended about 300 B.C. Whereas the Iberians were still in a Bronze Age civilization, the Celts had advanced into the Iron Age. The Celts' superior weapons, more powerful bodies, and warlike dispositions were more than the Iberians could cope with. The Iberians who survived were pushed into the western and northern wildernesses (Wales and Scotland). Eventually, a major Celtic tribe, the Brythons, came to be know as Britons and the island as Britain.