Δεν είναι κατά τι διαφορετικό; Αυτή την εντύπωση μου δίνει η Μπριτάννικα:
The army was a particularly important element within the empire. It, too, developed and changed with time. After Cyrus the Persian tribal levy, based on the responsibility of all male Persians to fight for the king, was replaced by a professional standing army supplemented by a troop levy from the subject peoples in times of intensive military activity. The elite of the standing army were the 10,000 “immortals,” composed of Persians and Medes, 1,000 of whom were the personal guard of the king. The person who controlled this elite guard, as did Darius on the death of Cambyses, usually controlled all. The troops of the imperial levy fought alongside the regular army in national units, were armed according to their individual customs, but were usually officered by Persians. Permanent bodies of troops were stationed at strategic points throughout the empire, and, to judge from the garrison at Elephantine in Egypt, these were actually military colonies, firmly settled into the local countryside. Greek mercenaries were used with increasing frequency in later years, and many Greeks fought faithfully for Persian silver.
http://www.britannica.com/EBchecked...nian Empire -- Britannica Online Encyclopedia
Αυτό που δεν μπορώ να βρω, όμως, είναι αν ένας "αυτοκρατορικός στρατός" λειτουργεί με τον ίδιο τρόπο - αν είναι δηλαδή το ίδιο πράγμα. Γιατί πλάι στο imperial levy βρήκα και το imperial army:
http://en.wikipedia.org/wiki/Imperial_Army