The most recent issue of Ancient Warfare magazine (X.6) contains an article on the battle of Chang-Ping in the Warring States period where allegedly several hundred thousand conscripts lost their lives. In western Eurasia, the first reliable evidence that anyone brought a hundred thousand or more combatants to a battle appears around the time of the Napoleonic Wars. (I could talk about what counts as reliable evidence, but suffice it to say that this is an empirical question and that numbers in stories about armies long ago and far away do not count). Occasionally one hears higher figures from India or China. Does any of my gentle readers know if those sizes are based on any real evidence, or just the usual choice between the various numbers given in stories about the battle?
One reason why I don’t think the possibility of Persian armies of hundreds of thousands of men is worth considering is that whenever we have good evidence, armies shrink to thousands and tens of thousands. This is true from the ancient Near East to the Greek and Latin tradition to the middle ages to the early modern period. Of course it is possible that some societies could have raised larger armies if they had wished to, just like it is possible that the Romans could have invented gunpowder and the steam engine. But in our world, the Romans got by with muscle-power, and armies for which we have records never crossed the line of 100,000 combatants until the Napoleonic Wars.
Some people cite theoretical calculations about the size of armies in the Chinese military classics, but the Strategikon of Maurice has theoretical calculations about armies of 300,000 cavalry (IX.3) while making clear that actual Roman armies rarely have as many as 20,000 (I.4, II.4, III.8, 10).
People who study late antiquity tell me that some writers start to give vast numbers for the forces of rebels long ago, such as “50,000 dead” at the battle of Mursa Major in 351 AD, or the army of emperor Maxentius with “188,000 men” in Zosimus II.15.2. I suspect that that this has something to do with the introduction of Christianity and the very large numbers in the Old Testament. Six hundred years ago, Ibn Khaldun noticed that those numbers could not be right! He observed that some armies in the Old Testament were much larger than the armies raised by much greater nations in recent times, and that it would be impossible for so many soldiers to line up and fight a battle in the conventional way.
Less often people quote vast numbers for Indian armies like the one which the Mughal Emperor Aurangzeb lead against Shivaji and the Marathas in the 17th century. But as I understand it, Indian armies of the period were loosely organized things, with large numbers of irregulars who came and went depending on whether the looting was good, and under the Raj and after independence the Mughal archives were allowed to decay. Estimating the size of armies like that is very difficult. Supposedly, almost no Mughal archives survive. In the history I know better, popular histories tend to quote numbers from literary sources without asking whether those numbers are true (although when sources disagree, they sometimes explain why they are choosing one number over another). So without a source which cites documents, or at least observations by an experienced soldier who was not trying to impress anyone, I am not terribly interested.
On the other hand, if someone has evidence for Indian or Chinese armies of more than 100,000 combatants (such as records of feeding and paying many men at a single place and time under a single commander) I would be glad to learn about it. I have no doubt that many Indian and Chinese rulers raised much larger armies than any European king in the same period could muster, I just doubt that those large armies numbered in the hundreds of thousands of men.
In my reading of world history, claims that an army numbered in the hundreds of thousands (or sixties of thousands in the Near East, where it was common to combine base 60 and base 10) is common. Greek writers attributed them to foreigners on the edge of the known world, the composers of the Old Testament to long ago, medieval chronicles attributed them to their own princes but especially to Arabs and Turks and Mongols and other outsiders, and renaissance ambassadors earnestly told their patrons that a certain country had hundreds of thousands of soldiers despite the fact that its actual armies numbered in the tens of thousands. There is only a single cuneiform source which claims that an army from the writer’s own kingdom in the writer’s own lifetime had that many, an inscription of Šalmanessar III of Assyria from the 9th century BCE. His boast that he crossed the Euphrates with an army of 120,000 troops (2 × šu-ši × LIM) is surrounded by other boasts which specialists in the Ancient Near East read with a grain of salt. When one collates different inscriptions and realizes that the same city was “burned and destroyed, leaving no-one alive” three times in five years, the suspicion emerges that the kings may have improved their deeds in retelling. Claims in stories that an army had 100,000 soldiers or more are common (especially if that army is from another culture or another age or was about to be defeated by the people telling the story). What is rare in world history is evidence that anyone managed to muster all of their military resources and bring that many soldiers through mountains and desert and fluxes and fevers and skirmishes and sieges to a battle.
Writers in the 20th century sometimes spoke as if 60,000 or 80,000 men was a small army. But in world history (as opposed to world literature) those are very high figures. Only in the 19th and 20th centuries were they temporarily eclipsed by the intersection of professional staffs, population growth, industrialized transportation, and mass conscription. As had happened before, most of the states which raised such armies eventually decided that they were expensive and socially disruptive and replaced them with small, highly-equipped professional armies. Since we now have nuclear weapons, it seems unlikely that the cycle will resume and someone will discover how to unleash the fury of the masses to overwhelm the skill of the professionals. But it is difficult to make predictions, especially about the future.
Further Reading: The works of Anne Curry on Agincourt and Michael Livingston and Kelly de Vries, The Battle of Crécy: A Casebook (Liverpool University Press, 2015) (available on Bookfinder) are the places to start for medieval and renaissance Europe. For Ottoman armies in archives and in literature, see Rhoads Murphy, Ottoman Warfare 1500-1700 (Bookfinder). For numbers in Assyrian inscriptions, see Marco De Odorico, The Use of Numbers and Quantifications in the Assyrian Royal Inscriptions, State Archives of Assyria Studies, Volume III (Helsinki, 1995)