Complex sentence: After Alfred the Great (who reigned 871--899) made the West Saxon kings the rulers of all England, the island became part of England, being of the shire of Hampshire and divided into hundreds as was the norm.
Split into simpler sentences: After Alfred the Great (who reigned 871--899) made the West Saxon kings the rulers of all England, it became administratively part of England. The island was then part of the shire of Hampshire and divided into hundreds as was the norm.