User blog comment:Tesla Man/Rise of the TDBU: Alexander the Great versus Xerxes/@comment-422690-20120320173747/@comment-2006360-20120323131917

The good thing about Xerxes' march through Greece was that he managed to recruit several tribes from north of Greece who had enmity with the city-states before, such as the Macedonians and Thracians. So he had a good force of mercenaries and allies on his side. It's kind of like Hannibal's strategy after him, to replace dead or deserted soldiers with the indigenous population who shared an enemy as him. (If you don't know who I'm talking about, it's the Celtiberian and Gallic barbarians.)