Complex sentence: At the age of twenty-two he was left by his father to defend Syria against Ptolemy the son of Lagus; he was defeated at the Battle of Gaza, but soon partially repaired his loss by a victory in the neighbourhood of Myus.
Split into simpler sentences: At the age of twenty-two he was left by his father to defend Syria against Ptolemy the son of Lagus. He was defeated at the Battle of Gaza, but soon partially repaired his loss by a victory in the neighbourhood of Myus.