Complex sentence: Gnaeus Pompeius and Sextus Pompeius would rally the Optimates cause in Hispania, where the Optimates would challenge Caesar for the last time at the Battle of Munda.
Split into simpler sentences: Gnaeus Pompeius and Sextus Pompeius would rally the Optimates cause in Hispania. The Optimates would challenge Caesar again at the Battle of Munda.