He was quite fortunate in his enemies. The Persian Empire is remarkable in the average length of the reign of its rulers, but the flip side of that is succession was invariably plagued with conflict. The new Great King, Darius III, seems to have possessed considerable personal courage, but because he had only come to power a few years before Alexander's invasion, and in a rather messy fashion, he had to be just as concerned about ensuring his own position as facing against Alexander, and Alexander himself had a rather easier time peeling off allies than he likely otherwise would have.

That aside, most of the people that discuss Alexander's success (like Peter Connolly) attribute it largely to his army and the officer corp created by Philip. Alexander himself was certainly a brilliant and inspiring figure, but he was also commanding an extremely effective military force.