Complex sentence: Initially a supporter of Mark Antony, Hirtius was successfully lobbied by Cicero, who was a personal friend, switched his allegiance to the senatorial party, and set out with an army to attack Antony, who was besieging Mutina.
Split into simpler sentences: Initially a supporter of Mark Antony, Hirtius was successfully lobbied by Cicero (who was a personal friend) and switched his allegiance to the senatorial party. He then set out with an army to attack Antony who was besieging Mutina.