Complex sentence: After the successful conclusion of the Siege of Saguntum, Hannibal Barca had rested his army, then in the summer of 218 BC had started out for Italy with either 90,000 foot and 12,000 cavalry (according to Polybius), or with 46,000 foot and 10,000 horse.
Split into simpler sentences: After the successful conclusion of the Siege of Saguntum, Hannibal Barca had rested his army. Then, in the summer of 218 BC, he had started out for Italy with either 90,000 foot and 12,000 cavalry (according to Polybius), or with 46,000 foot and 10,000 horse.