Complex sentence: The Dukes of Brittany suffered continuing Norse invasions and Norman raids, and Brittany lost the Cotentin Peninsula (and Avranchin nearby) after only 70 years of political domination.
Split into simpler sentences: The Dukes of Brittany suffered continuing Norse invasions and Norman raids. Eventually the Cotentin Peninsula (and Avranchin nearby) was lost to Brittany after only 70 years of political domination.