How was Alexander able to defeat Darius?

Preceding his invasion, a period of continuous skirmishing and political intrigue between Persia and the Hellenic city-states had prevailed up to the assassination of Alexander's father, King Philip II of Macedon, in 336 bc. Although the person responsible for Philip's murder was never conclusively determined, many historians regard his divorced wife, Olympias, princess of Epirus and mother of Alexander, as the most likely suspect.

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