Britain was a very different place in the chaotic centuries after the Roman legions left in 410 CE. With the breakdown of the centralised Roman administration, the country dissolved into innumerable petty kingdoms, many now completely lost to memory, contending with each other for short-lived dominance. Into this mix came the Anglo-Saxons, sailing over the North Sea and using the rivers and estuaries of east and south Britain as their highways into this new country where they were carving out kingdoms. Roads were few and often dangerous. The sea and rivers provided much surer and safer means of travel. These bands of warriors established new kingdoms and brought their families over the North Sea to join them, but they fought as enthusiastically among themselves as they fought with their British neighbours.
Slowly, the smallest kingdoms were swallowed up, incorporated into larger realms and as a victorious king could give more gold and glory to warriors who came to his court, this process became self-reinforcing, leading to the gradual amalgamation of all the smaller kingdoms until there were seven, and finally four, Anglo-Saxon kingdoms in England.