At first the Achaemenid army consisted wholly of Persian (Iranian) warriors, and even when other regions were subjugated, Persians formed the nucleus of the imperial army. With the expansion of the once tiny kingdom of Persis into a world-empire embracing all Persian groups, from Central Asia to the Danube, a standing army was formed from Persians, Medes, and closely related peoples, and an imperial army was organized by incorporating warriors of all subject nations.
Military documents ultimately used by Herodotus prove that the closer a nation was to the Persians, the more it shared in the domination of the empire by paying less tribute but contributing more soldiers. Thus, the Medes who had the second position in the empire furnished more soldiers than others and indeed many of the imperial generals were chosen from the Medes (Mazares, Harpagus, Taxmaspada, Datis, etc.). Then came the Sacians, Bactrians, Hyrcanians, and other East Iranian groups.
Apart from the standing army, the rest of the levies were recruited when the need arose, and it took a long time, sometimes years, to muster a grand army. There were many Iranian garrisons in important centres of the empire, and satraps and governors also had their guards and local levies, but these could not be depleted to form an army on short notice because the danger of revolt was always present. Tribal troops, especially from eastern Persia, were more readily available. Levies were summoned to a recruiting station (handaisa) where they were marshalled and reviewed. In the time of Darius & Xerxes a grand army could contain as many as 50 or more ethnic & national contingents.
The general term for the professional army was spada. This consisted of infantry (pasti), cavalry (asabari “horse-borne,” and occasionally usabari “camel-borne”), and charioteers (only the noblest warriors used the then obsolete but symbolic chariot), and a large number of camp followers. From the moment they met the Greeks, the Persians incorporated subject or mercenary Greeks in their army. As the time went by, not only Persian satraps in Asia Minor but also the King of Kings employed Greek mercenaries, each of whom received free board and a monthly wage. At the great battles of Salamis in 480bc, and Plataea in 479bc, more Greeks fought with the Persian Army than against it. By the time of Alexander the Great, these mercenaries had become a regular part of the spada and their leaders had been incorporated into Iranian aristocracy. Again during Alexander’s battles with Persia, more Greeks fought against him than for him.
The organization of the spada was based on a decimal system. Ten men composed a company under a dathapati; ten companies made up a battalion under a thatapati; ten battalions formed a division under a hazarapati; and ten divisions comprised a corps under a baivarapati. The whole spada was led by a supreme commander (probablyspadapati, although a general with full civil authority was called karana (from the Greek “karanos”), who was either the King of Kings himself or a trusted close relative or friend (e.g., Mazares the Mede led Cyrus the Great’s army and Datis the Mede led the army of Darius the Great at Marathon). A characteristic of the Achaemenid period is that commanders and dignitaries participated in actual fighting, and many of them lost their lives in action.
The foot soldier carried a short sword (acinaces), a spear with wooden shaft and metal head and butt, a quiver full of arrows of reed with bronze or iron heads, and a bow about one meter long with ends formed in animals’ heads, and a case which combined the bow- case and quiver-holder. A symbol of kingship and the Persians national weapon, the bow was held in the hand of the King of Kings on his tomb and on coins. Battle-axe was also used, especially by northern Persian tribes. For protection, the infantryman relied on his wicker shield (made of sticks threaded through a wet sheet of leather capable of stopping arrows). The shield was either small and crescent-shaped or large and rectangular; the latter could be planted in the ground allowing the archer to fire his arrows from behind it, whilst protected further by a spearman behind him. Some guards carried the large “figure- of-eight” -shaped shield known as the Boeotian, while the Gandharans carried round shields not dissimilar to those of Greek hoplites. Some Persians wore metal helmets, but, other than the Imperial Guard units (immortals) only the Egyptians and the Mesopotamian contingents wore armour for body protection.
The elite infantry had varied costumes: either the fluted hat, short cape over a shirt, pleated skirt and strapped shoes of the Elamite court type, or the conical felt hat, tight- fitting tunic and trousers and boots of the Median cavalry type. One division of the infantry comprised “one thousand spearmen, the noblest and bravest of the Persians” who formed a special royal guard; their spears had golden apples as butts from which they were called the “Apple-bearers”.
As a prince, Darius the Great served in this guard of spearmen under King Cambyses. Their commander was the hazarapati of the empire, who, as the officer next to the emperor, possessed vast political power. All members of this guard fell at Plataea defending their position.
One corps of the spada consisted of ten thousand elite Iranian foot soldiers, the so-called “Immortals” whose “number was at no time either greater or less than 10,000” – Herodotus. These had variegated costumes and acted as the Imperial Guards. “Of these one thousand carried spears with golden pomegranate at the lower end instead of spikes; and these encircled the other nine thousand, who bore on their spears pomegranates of silver” – Herodotus
One of the main weapons of an Immortal was the short bow which was of little use against the heavy armoured Greek hoplites. His other main weapon was quite a short spear with an iron point and a silver counterbalance (the officers had a golden counterbalance). A short sword, or a big dagger, completed the weaponry of an Immortal. A corset with metal plates is worn under his tunic to offer some protection. In his left hand he is holding the gerron: a traditional shield made of wicker and leather. This shield gave him enough protection against arrows, but it certainly could not stop a well aimed thrust of a spear of a Greek hoplite while the Greek hoplon could stop such an attack. On his head he is wearing the tiara: a soft cap made of fabric which he could pull down over his face when he marched through a desert. His loose tunic is richly decorated with embroidery and was often painted in purple, blue, yellow, or white.
Campaigns usually started in early spring. Provisions were stored at various magazines along the route of the army, and were also brought with it in baggage-trains. Royal and religious emblems accompanied the centre of the army where the commander had his position: the eagle standard and the holy fire in portable fire-holders attended by Magi chanting hymns, and the sacred chariots of Mithra, Ahura Mazda and others. Mounted scouts were sent in advance to watch the enemy’s movements. There was also an excellent system of communication: couriers on the royal road changed horses at short intervals and speedily conveyed their messages to their destinations; also by their light and mirror signals the King of Kings in Susa and Ecbatana received the news from the whole empire, it is claimed, on the same day. Fire signals communicating the news from towers and heights were widely used with good results. Fortified gates were set up in narrow passes leading into various provinces not only for custom checks but also for stopping the advance of an enemy.
The Imperial Achaemenidian Army was undoubtedly well-organised, equipped, supplied and supported and could call upon vast resources in terms of man-power and finances. It was also undoubtedly well-suited to the type of warfare common in the eastern world, dominated by light troop types, archery and cavalry.
It’s consistent failure to beat Greek heavy infantry-dominated armies, despite huge numerical advantages, was because imperial generals failed to use their advantage in missile weapons and cavalry (both of which Greek armies lacked), and they generally allowed the Greeks to chose battlefields that suited their heavy infantry troop types and tactics.Persian battlefield loses at Marathon, Thermopylae, Plataea and numerous others is testament to this.