Complex sentence: Sergius was forced to acknowledge him as overlord of Naples and switch his allegiance to Anacletus: that moment marked the fall of an independent Neapolitan duchy, and thereafter the ancient city was fully integrated into the Norman realm.
Split into simpler sentences: Sergius was forced to acknowledge him as overlord of Naples and switch his allegiance to Anacletus. This moment marked the fall of an independent Neapolitan duchy, and thereafter the ancient city was fully integrated into the Norman realm.