What were the main factors which contributed to Alexander conquering the Persians?

[Brennus]

I agree with MisterFred the recent civil wars did contribute to a large extent and the well crafted Persian system of government played well into the hands of Alexander.

Personally I doubt Alexander's generalship, some of his decisions seemed very reckless and some of his decisions show a lack of concern for his troops wellbeing.

I still think he was a very good general, but not the best antiquity ever produced.