Not precisely You can say Persian Empire crumbled after battle of Gaugamela when Mesopotamia was taken or after Bessus killed Codomannus but battle at Persian Gates is completely irrelevant in context of Achaemenid fall You re right part of Greeks have leaved Thermopylae part of them surrendered one of them collaborated and part of them fought till death You can t accuse me for anti Hellenism because I ve edited many Greek pages Battle against Porus was Macedonian victory but also not decisive victory It s nonsense to say Alexander did conquered Persia but Persians didn t conquered Greece fact is Persia fought wars only against Athens and Sparta and their minor allies and they razed Athens during Xerxes invasion Perhaps you didn t know but Macedonians sided Persians during that expedition Alexander is not Greek in ancient sense but Macedonian also not in modern sense but ancient Sparta didn t participated in Macedonian invasions of Persia while Athens and other south Greek polises were just Macedonian vassals part of Corinthian league 93 142 189 185