Complex sentence: These were left behind as Darius fled away in defeat at the Battle of Issus but were secured by the general Parmenion sent by Alexander, by whom she became the mother of a son named Heracles.
Split into simpler sentences: These were left behind as Darius fled away in defeat at the Battle of Issus but were secured by the general Parmenion sent by Alexander. After Barsine and Alexander were brought together by Parmenion, she became the mother of a son named Heracles.