Complex sentence: John was only an infant when his father died and he was quickly deposed and exiled by the Amalfitans, who required a ruler who could defend them and so surrendered to the Normans led by Robert Guiscard.
Split into simpler sentences: John was only an infant when his father died, The Amalfitans, who required a ruler who could defend them, quickly deposed and exiled him. Consequently, given the lack of an adult ruler, Amalfi surrendered to the Normans led by Robert Guiscard.