There is no doubt that Alexander the great, son of Zeus himself, wouldn't be [I]that [/I ]great without the preparations made by his father, Philip. About why he chooses the Persian Empire, well, what I would say is that it was the only obviously threat, this was where the challenges and riches could be found. His father was about to invade the Persians, maybe he got his father killed so the awaiting glory would be his? Who knows? It had always been the Greek dream to go East, though the Macedonians were not Greeks, but they tried. I guess this dream had been adopted by them as well. There were many reasons, I'm not sure if there exist a 100% correct answer.

Cheers.