Saturday, September 22, 2007

How Large Was Xerxes' Army?

At Brett Schulte’s new Civil War group blog, TOCWOC (The Order of Civil War Obsessively Compulsed - Informed Amateurs Blog the American Civil War), Ray B has a fun post in which he applies the lessons of Thermopylae to the Civil War.

As I usually do, I’ll only pick at the details. Ray says:

A total force of some 7000 Greek and Spartan allies was able to hold off an invading force of perhaps millions. (Xerxes’ Persian army consisted of a little over 5 million, but not all were present at Thermopylae. Estimates range anywhere from 300,000-800,000 actually took part in this battle.)

Let’s look at the Persian numbers in greater detail.

In his History, Herodotus tells us that Xerxes’ infantry numbered 1.7MM men. There were also 100,000 cavalry, camel corps and chariot corps, plus 300,000 Thracians and Greeks (more than the entire population of the Balkans!), for a total of 2.1MM. There were 541,610 men manning the fleet, for a total armed force of 2,641,610 men. He then adds non-combatants (cooks, drivers, etc.) to double the total and reach the awesome number of 5,283,220.

Demographics, logistics and common sense demonstrate that these numbers are impossible. The most attractive theory that both explains how Herodotus could have come up with these numbers and suggests the true order of magnitude of Xerxes’ forces lies in the structure of the Persian army. The army was organized in a decimal system up to units of ten thousand. Units of ten men were formed into units of one hundred; units of one hundred into units of one thousand; and units of one thousand into units of ten thousand men. The 10,000 men units were known as Myriads, under the command of Myriarchs. The Myriads were then organized into corps of 60,000 men.

Herodotus understood there were 30 corps commanders, 29 of whom commanded 60,000 men each. He made his basic calculation of 1.7MM infantry simply by multiplying the number of corps commanders by 60,000.

But what if Herodotus (who did not speak Persian, after all) or his sources had confused the Myriarchs and the corps commanders. 30 Myriarchs times 10,000 men each would produce an infantry force of 300,000 men.

This number is still unreasonably high, but at least it is in the right ballpark. Other other considerations may get us closer to an actual figure.

The first is the difference between actual strength and nominal strength. As any student of the United States Civil War knows, there is often a dramatic difference between the two figures. Even without fighting a battle, tens of thousands of men crowded together create unsanitary conditions that in turn generate disease. Assuming that each Myriarch started with a full complement of 10,000 men (and that in itself is a questionable assumption – was there no recruiting fraud in Persia?) those figures would have been quickly reduced.

Second, Xerxes' army clearly included both regular troops and more lightly armed allies. By analogy to the later Roman army, the allied units may well have been smaller. In short, many of Herodotus' thirty commanders may have been commanding units of, say, 5,000 men rather than 10,000.

Finally, it is not credible to believe that the entire army simply marched to the front. Xerxes’ commanders would have left sizable contingents along the march, to guard fords and bridges, man forts, protect supply depots, and the like.

Historian A.R. Burn has pointed out that the limited water sources along the route, if nothing else, "lead[] to the conclusion that 200,000 is about the limit for the total manpower of the expedition." All in all, I would guess there were substantially fewer than 200,000 Persian infantry -- perhaps somewhat more than half that number -- in the neighborhood of Thermopylae when the Persian army encountered Leonidas and his colleagues. It's not 5,000,000, I know -- sorry to disappoint -- but still a huge number when you consider that 2,400 years later large Civil War armies were in the range of 100,000 men.


  1. Howdy, fellow classics major here. My parents want me to become a lawyer too but I'm still not convinced :P
    I always thought those numbers were a bit funny... I'm currently reading Thermopylae: Battle for the West by Ernle Bradford. Great read if your interested.
    I read that the forces of Xerxes even without Herodotus' exaggeration may have been capable of drinking a lake dry.
    Here's my classics blog if your interested:

  2. Οι ΗΠΑ, η Ρωσία και η Κίνα στην Μέση Ανατολή :
    Συνεργασίες & Συγκρούσεις


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