So the subject of the size of Xerxes' armies is one of neverending controversy and easy miscommunication. There was a discussion a while back involving me, /u/Iphikrates and /u/wotan_weevil , which I can't locate at the moment. The lower bound for the size of Xerxes' army is constrained by the more easily estimated army at Plataia, around 40,000-60,000 troops. The upper bound, logistically, is perhaps around 160,000 or so, with a more plausible range being around 80,000-120,000 fighting men - still enormously large. The reason for why I have become inclined to lean toward the plausbility of such a bloated army is because severe logistical problems would help explain Xerxes' retreat and Mardonius' march with a leaner army, which isn't necessarily adequately explained by the naval defeat at Salamis.

The first thing we need to do is to consider the sheer size of the Achaemenid realms. Demographic projections are a bit of a sketchy trade, but we're looking at a population on the order of 30 million people, with millions in Anatolia alone. The taxation reforms of Dareios had made the Great King's court absurdly wealthy, abled to undertake absolutely massive construction projects. By comparison, William ruled a Duchy which by my back-of-the-envelope estimate (10 million people in France, ~10% of the population) had perhaps a million inhabitants.

Our ability to understand the means by which the Achaemenids raised armies are, mildly put, limited, but we have a somewhat better understanding of the political culture. Power in the realms was essentially synonymous with kinship and favour of the Great King; the mere presence of the monarch appears to have guaranteed that practically the entirety of any of the 20-odd Satrapies' resources were at his disposal, including the personal wealth of the Satrap, in anticipation of royal favour. The Great King therefore had both a massive population base and a large latitude in throwing his weight around, making his wealth and manpower practically unlimited resources. There would not need to be any sort of exceptional mobilization of the population (as there was in the Greek city states) for his armies to reach absolutely enormous scales.

Needless to say, the two cases are simply not comparable.

There are several ways to look at this and many reasons for it, but in essence, medieval armies were professional forces comprised of landed gentry, mercenaries and career soldiers, whereas the armies of antiquity were comprised of the bulk of a city's male population, in the case of the Greek city-states, or huge levies derived from across an entire nation e.g. the Persian Empire. Every man of fighting age, being physically fit and not otherwise engaged to a more important posting, would march off on campaign. This meant that battles and sieges would be conducted on a massive scale.

During the Second Punic War; when Hannibal Barca put the Roman's to the sword at Cannae, Rome lost 10% of its population in a day. The reason ancient warfare involves such large numbers is a point I'll address in more detail later on, but it fundamentally stems from a greater sense of national unity, the technological and societal development of the time and as a pragmatic answer to conquering a city.

​

While we often associate medieval armies with peasantry; by the 15th century, the use of peasant levies and serf conscripts had fallen away to be replaced by the much more effective trained troops. The reason for this comes down to 2 main points. Feudalism and war objectives.

Feudalism breeds a very divided society and results in a very particular style of warfare. The goals of various campaigns typically ranged from capturing and ransoming high value targets, to looting and garrisoning settlements and fortifications. It was as much about profit and prestige as any nationalist ideals, often more so. This being the case, the army a single lord or king could bring to bear would be entirely reliant upon the whims and interests of the various noble factions sworn to him and the relative state of their finances. To gain support, wars would have be brief and profitable, with clear aims of sacking valuable targets and claiming ransoms. Rarely was the goal the wholesale occupation and annexation of another state (though of course, there are examples of this.)

In the ancient world, whole civilisations waged war upon each other. And whilst we could adopt a Marxist interpretation and state these wars were fundamentally fought for the same economic reasons, I think that simply fails to address the clear and contrasting differences between these types of warfare. Emperor Xerxes and his contentiously debated 250,000 men were not solely focused upon sacking cities and ransoming captives, though these were obviously incentives in a every pre-modern war; they wished to conquer. To assert the will of the Persian dynasty and bring Greece, as a civilisation, into the envelope of their hegemony.

​

On Sieges

In the medieval period, armies besieged castles and walled towns. Castles are designed to be held by as few men as necessary, a great example of this is Lady Mary Bankes defending Corfe Castle during the English civil war. She successfully withstood a 3 year siege and hundreds of attackers, with just herself and 5 men. My point is that you don't need hundreds of thousands of men to attack a castle, especially with the rise of trebuchets and early forms of artillery. Likewise, even when assaulting a town, whilst you could expect a militia to be raised from the populace, these common people would be unable and unwilling to stand against professional soldiers in a protracted engagement. Ergo, a smaller force of disciplined professional soldiers can achieve great success without requiring the vast array of food, wages and supplies a larger host would. To bring it back to Feudalism, this also meant a far larger portion of a lord's tenants would be able to stay at home and work the land for profit and food.

​

In antiquity, armies besieged cities. The difference being, a city has vastly larger reserves of supplies and defenders. If a city's population was in the region of 400,000, they could raise a substantial defensive force. An attacking army of some 14,000 men, professional soldiers or otherwise, would be simply unable to oppose them. Therefore an attacking army of tens or hundreds of thousands of men are required to be able to operate in the theatre.

The relative sizes of these types of armies results in two very different kinds of siege craft. Medieval sieges were long and drawn out affairs, during the 17th century the Ottomans besieged the Venetians at Canida for 22 years.

In comparison, ancient sieges were typically brief things (though not always). The primary reason being that if you've got a force of over a hundred thousand men at your disposal, you can construct vast earthworks in relatively short amounts of time. An agger is essentially a dirt ramp that the attackers would build up the walls and then simply walk across. Obviously, this kind of tactic isn't feasible with an army of 10,000; which is why we see siege towers, battering rams and early artillery utilised during the medieval period.

​

The Crusades

As a topic, the Crusades are worthy of several books in their own right, but I feel there's a relevant point I must briefly address. The Crusades in the middle-east, in their entirety, do not follow this formula of medieval dynastic struggles and regional conflicts (Though I think there is an argument to be made that the Crusades in eastern Europe were more akin to the conventional warfare of the time).

The middle-eastern campaigns are unique in terms of their objectives, the multi-national composition of their armies, the strategies they employed and how they came about. It is often easier to force history through a modern lens and dub something like the Crusades an early form of colonialism or just more of the same war profiteering, but as ever, simple readings of history rarely stand up to scrutiny.

I won't stray any further down this tangent, but will state that they exist in their own unique place in history and were fundamentally atypical of the European wars of the time.

​

​

For every point I made, you could cite an example displaying the opposite. The history of warfare is long and storied and the motivations and tactics involved are myriad; this is more of a brief analysis of the general methods and mechanics of war in these 2 eras as opposed to a set of rules and absolutes.

​

TL;DR - Medieval armies were small forces comprised of professional soldiers involved in regional conflicts and dynastic struggles. The armies of antiquity were comprised of the majority of a city-state's male population, or from vast levies raised throughout a nation, and were used in wars of conquest and in defence of entire civilisations.