How long did it take Alexander the Great to conquer the Persian Empire?
From 334 to 327 BCE.
Because it was bequeathed to him by his father Philip II of Macedonia. The plan was Philip's, but on the eve of his departure he was assassinated. His son Alexander then took over the project. Philip decided to conquer the Persian Empire because, having brought mainland Greece under control, Persia offered a target for his boundless ambition. His son Alexander inherited boundless ambition.