Why the notion of 'first professional army' in the world it is atributed to the romans when the macedonians under Phillip (father of Alexander) introduced full-time occupation in the military much earlier. The roman legionaries were more 'professional' than macedonian soldiers of Philip and Alexander's ?
If I'm allowed to tag along, would the Immortals of the Meads/Persians count as a "professional army"? IIRC, the persian elite core existed since Cyrus the Great. Is there any recommended reading for an introduction to the unit's (organization's?) history?