Does make you wonder why the Persians never really adopted some form of heavy infantry wholesale... Especially after their experience with Greek hoplites during the Greek-Persian wars. Although I suppose they simply employed them as mercenaries, which may have been quicker and/or cheaper than training there own men/units.

Mind you the way the Persian army was composed it had conquered a large chunk of the known world by this point and had kept the Ionian Greeks at bay for centuries. Their army as it was had not only conquered, but maintain the status quo and I imagine was not seen as needing re-forming until well Alexander came along and by that point it was too late!

I guess you also have to bear in mind that it wasn't a walk in the park for the Macidonian army either and that they very nearly could have lost their engagments with the Persian army. I imagine luck would have played a big part in the outcome also...

Suppose I'm answering my own questions as I carry on with this!