Complex sentence: He was greatly influenced by his co-writer supported by Scipio Aemilianus Polybius, who attempted not only to record events as they took place, but also to look for the causes that led to them.
Split into simpler sentences: He was greatly influenced by his co-writer Polybius, who was supported by Scipio Aemilianus. Polybius attempted not only to record events as they took place, but also to look for the causes that led to them.