After the Battle of Gaugamela, Darius III and what remained of his armies fled eastwards into the Upper Satrapies, leaving Babylonia behind. The satrap of Babylonia, Mazaeus, quickly defected to Alexander and Darius III was murdered by his own generals. What if he fled southwards into Babylonia, with the survivors of Gaugamela, to organize a defense of Babylon? How would the conquest of the Achaemenid Empire go differently from OTL? Could stronger resistance force Alexander to stop and consolidate his gains?