The army of the Achaemenids changed over time from being that of a specific people to that of a universal empire. Under King Cyrus II the Persian army was a tribal levy of all male Persians only. In King Darius I's reign this system was replaced by a regular army which was reinforced in times of war by a troop levy of all subject peoples.
These troops were formed into national units which were armed and dressed according to their own customs, but which were led by Persian officers. Some of the regular troops were stationed permanently at key controlling points throughout the empire.
In later times, Greek mercenaries featured prominently in Persian armies, fighting loyally for their employers even when faced with other Greeks as opponents, as at Granicus where they fought against the troops of Alexander the Great.
The most famous component of the Persian army was the force of the 'Immortals' - a group of 10,000 Persian and Median soldiers. The finest 1,000 of these were chosen to be the king's bodyguard. They had an excellent fighting reputation and were much admired by enemies and allies alike.
