Complex sentence: For a brief period in the 8th century the kingdom also encompassed the Kentish Kingdom to the South, but by the mid 8th century much of the kingdom, including London, had fallen to Mercia.
Split into simpler sentences: For a brief period in the 8th century the kingdom encompassed the Kentish Kingdom to the South. However, by the mid 8th century much of the kingdom, including London, had fallen to Mercia.