Complex sentence: After the Romans left Britain in the 4th century, the Warwickshire area was settled by Anglo Saxon tribes and by the 8th and 9th century became a part of the kingdom of Mercia.
Split into simpler sentences: After the Romans left Britain in the 4th century, the Warwickshire area was settled by Anglo Saxon tribes. By the 8th and 9th century, the Warwickshire area had become a part of the kingdom of Mercia.