Perhaps the greatest controversy relative to the Battle of Thermopylae of 480 B.C. is the issue dealing with the number of warriors which Herodotus claimed comprised the Persian army. Conversely, the size of the Greek army at Thermopylae has seemed less prone to exaggeration, therefore, it has appeared to be much more credible.
The translation of an epigram written by Simonides which appears below lends credence to the number of Greek warriors:
Against three million men fought in this place
Four thousand Peloponnesians, face to face.
The number which comprised the Peloponnesian contingents consisted of:
200 warriors from Phlius
This amounts to 3,100 warriors, which is less than the 4,000 inscribed on the epitaph. However, the difference of 900 is most likely to be the emancipated Helots who accompanied the Spartiates in the ratio of 3 to 1. This would account for the 4,000 Peloponnesian warriors to which Herodotus records another 700 from Thespiae and another 400 from Thebes. The historian Diodorus also stated that a 1,000 Locrians and another 1,000 from Phokis increased the ranks to approximately 7,100 Greek warriors.
Persian archer - Courtesy of Nikos Panos
Now as far as the Persian army is concerned, therein lies the greatest dispute. How was it feasible for Xerxes to amass an army of 2.1 million which consisted of infantry, cavalry, charioteers, Thracians, etc.? The logistics (water, food, etc.) alone show that it was highly improbable in coordinating the mass movement of troops through Asia & Africa into Greece.
Perhaps the most plausible theory as to the size and composition of the Persian army can be found in Peter Green's book 'Xerxes at Salamis' which was later retitled as 'The Greco-Persian Wars'. In his book, Professor Green notes that "The Persian command structure operated - at least, up to corps level - on the decimal system."
It is possible that Herodotus inadvertently recorded the strength of the Persian army by tenfold since he was not present to witness the events of 480 B.C. Therefore, he had to rely on the narratives of others which may account for the exaggerated numbers. Instead of 2,100,000 combatants in the Persian army, this number could have translated to 210,000 warriors. This would be a more realistic number, however, it still should be emphasized that the Persian army overwhelmingly outnumbered the Greek garrison led by King Leonidas and his 300 Spartans.
Therefore, the magnitude of the Persian army is believed to have been overstated not only by Herodotus, but also in 20th Century Fox's 'The 300 Spartans' and Warner Bros. '300'. However, '300' with the aid of computer graphics imagery which can be seen via the image below, has been able to duplicate the essence of the overwhelming numerical superiority which the Greek garrison faced.
The Persian army in the Warner Bros. movie '300'
The size of the Persian army has perplexed historians and scholars alike and it continues to this day. Since the numbers of men amassed by Xerxes for the invasion of Greece are impossible to quantify with any certainty, a range is probably the best way to assign a number.
In A.J. Munro's 'Some Observations of the Persian War' which was published in 1902, Munro stated that the six generals of the Persian army each had 60,000 men under his command. Therefore, the generals, Mardonius, Tritantaechmes, Smerdomenes, Masistes, Gergis and Megabyzus had 360,000 infantry and calvary amongst them, however, this does not include the 10,000 Immortals commanded by Hydarnes. On the other hand Julius Beloch believes that the number is as low as 60,000, therefore, the numbers which historian Peter Green calculated as 210,000 men in his book 'The Greco-Persian Wars' appear to be the most credible.