Several of the Germanic tribes(the most amongst no Angles, the Saxons and the Jutes) migrated across the sea now known as the English Channel to the British Isles.
There they were confronted by the Celts, the original inhabitants of the Isles.
The Celts desperately defeated their lands against the invaders, but they were no match for The military-minded Teutons and gradually yielded most of their territory.