Size of the Persian army

Xerxes I, king of Persia, had been preparing for years to continue the Greco-Persian Wars started by his father Darius. In 481 BC, after four years of preparation, the Persian army and navy arrived in Asia Minor. A bridge of ships had been made at Abydos. This allowed the land forces to cross the Hellespont. Herodotus of Halicarnassus, who wrote the first history of this war, gave the size of Xerxes's army as follows:

This is the account for the land armies present at Thermopylae. Regarding the total number of forces Xerxes assembed to invade Greece (land army, fleet crew etc), this number needs to be nearly doubled in order to account for support troops and thus Herodotus reports that the total Persian force numbered 5,283,220 men, a figure which is regarded erroneous by modern estimations. The poet Simonides, who was a near-contemporary, talks of four million. Ctesias of Cnidus, Artaxerxes Mnemon's personal physician, wrote a history of Persia according to Persian sources that unfortunately has not survived, and gives 800,000 as the total number of the original army that met in Doriskos, Thrace, after crossing the Hellespont. Modern scholars have given different estimates based on knowledge of the Persian military systems, their logistical capabilities and supplies available along the army's route.

Modern estimations tend to consider the figures given in ancient texts as miscalculations or exaggerations on the part of the victors. It is assumed that if Herodotus' 300,000 estimate at Mycalae were to be accepted, then the land army at Thermopylae could not have surpassed 500,000, and the total Persian presence in Greece would be estimated at 1,000,000. This accounts for one fifth of Herdotus' record. Others give an upper limit of 250,000 total land forces and 500,000 for the expedition. The main reason most often given for these values is a lack of water; Sir Frederick Maurice,] a British general in World War I, was among the first to claim that the army could not have surpassed 175,000 due this reason. A minority of scholars have suggested land force figures lower than 100,000, while a popular view supports a range of 100,000-150,000 or 150,000-200,000. The subject has been hotly debated, but the current consensus rests on the theory that Herodotus confused Persian terms for chiliarchy and myriarchy (one thousand and ten thousand). This suggests that the actual size of the Persian land forces would be around 210,000. All those estimates concern the land forces alone, whereas the entire Persian presence, including support troops and fleet crew, would almost double this number, dividing Herodotus' five million figure by ten.

The numbers given by Herodotus on the Persian fleet are considered largely realistic. It is generally maintained that Herodotus or his sources had access to official Persian records of the forces involved in the expedition, and it is more likely the numbers on the fleet were given precisely, whereas the contigent of the army may have been listed in general terms rather than exact figures. Whatever the real numbers were, it is clear that Xerxes was anxious to ensure a successful expedition by mustering an overwhelming numerical superiority by land and by sea.

Based on the fact that Xerxes led a multi-ethnic army and not just a Persian one, a second school contends that some ancient sources do give realistic numbers. According to the texts the Greeks at the end of the battle of Plataea mustered 110,000 (Herodotus) or 100,000 (Pompeius) troops: 38,700 hoplites and 71,300 or 61,300 peltasts respectively, the difference probably being 10,000 helots. In that battle, according to Herodotus, they faced 300,000 Persians and 50,000 Greek allies. This gives a 6-to-1 ratio for the two armies, which proponents of the school consider a realistic proportion.

Furthermore, Munro and Macan argue for realism based on Herodotus giving the names of 6 major commanders and 29 μυρίαρχοι (muriarchoi)—leaders of the baivabaram, the basic unit of the Persian infantry, which numbered about 10,000 strong. As troops were lost through attrition, the Persians preferred to dissolve crippled baivabarams to replenish the ranks of others. It is therefore likely that the units were at full strength. Adding casualties of the battles and attrition due to the need to guard cities and strategic objectives obtains a force of 400,000 minimum.

According to this view, there was no lack of water. The available surface water in Greece today satisfies the needs of a much larger population than the number of Xerxes's troops, though the majority of that water is used for irrigation.

Other historians' estimates include:

• Nicholas Hammond accepts 300,000 Persians at the battle of Plataea, though he claims that the numbers at Doriskos were smaller.
• The metrologist Livio Catullo Stecchini argues that Ctesias's figure of 800,000 battle troops for the Persian army is accurate and that Herodotus's figure of 1,700,000 includes both battle and support troops.
• Dr. Manousos Kampouris argues that Herodotus' 1,700,000 for the infantry plus 80,000 cavalry (including support) is realistic for various reasons including the size of the area from which the army was drafted (from modern-day Libya to Pakistan), the lack of security against spies, the ratios of land troops to fleet troops, of infantry to cavalry and Persian troops to Greek troops.
• On the other hand, Christos Romas believes that the Persian troops accompanying Xerxes were a little over 400,000.

Size of the Greek army

According to Herodotus, the Greek army included the following forces:

To this number must be added 1,000 other Lacedemonians mentioned by Diodorus Siculus and perhaps 800 auxiliary troops from other Greek cities, bringing the total up to 7,000. Diodorus gives 4,000 as the total of Greek troops, and Pausanias 11,200. Modern historians, who usually consider Herodotus more reliable, prefer his claim of 7,000 men.