You'd be surprised.

We do for example have records of what 'in theory' Napoleon was giving his troops in the early 19th century and it isn't far off the numbers I listed. This is the 'ideal' situation, not when they were marching home from Moscow:

24 oz of bread, 8 oz of meat, 2oz legumes.

My own numbers were more bread focused with less meat, but on that note you saw how pissy some folks got when I suggested half that quantity of meat upthread.

Realistically it's all pretty low. Would they need more water? For sure. But 1.5L would likely keep them alive, especially if they were campaigning during the cooler, non growing seasons which is the only way you'd ever get your numbers to even approach 100 or 300 thousand as you would be relying on your farmers as the bulk of those troops.

The bar I set is not 'well fed'. 2500 calories is what I would consider to be survival level given the physical expectations of an army on the march and in battle. For reference, a modern American 24 hour pack of MRE's contains between 3500 and 4000 calories. Likewise, water I estimated at a low end, since it is quite likely that an army on the move would almost certainly follow a river, where they could access flowing water that could be used for cooking and even be somewhat 'purified' for drinking through the addition of alcohol or vinegar.

Right, which is why I went for 2500 calories. For reference, a modern MRE averages at 1250 calories per meal, or 3750 calories per day.

I picked on red meat as a straight forward protein source that every ancient society would have at least had access to. It's also why I counted 2200 of their 2500 calories as coming from bread.

Were there other options? Sure. Beans and other legumes would fit the bill and would have likely played a significant role but beans also would have needed more water, which would have further complicated things. Plus, not every society would have had access to the same sorts of legumes so in the spirit of keeping it simple I put the number at 4oz of meat since going into more detail than that is just going to get too situational. As a note Tacitus does speak briefly as to the meat requirements of a Roman legion numbering it at around 150 swine/day for a full legion of 6000 troops plus their supporting folks. Now he was speaking about a legion in garrison, not on the march and we have no idea if it was some super special 'Legate Picard Day' or something but the point remains, that's a hell of a lot of food.

I also left out fat requirements since the Romans solved this through olive oil but I have no frigging clue how the Assyrians or Egyptians would have done the same outside of imports.

Good point, but even then if you apply the same logic as I did in my other post regarding food requirements to say the Battle of Kadesh where (once again according to Wikipedia) there were supposedly 20,000 troops on the Egyptian side... Now this was an army operating out of Egyptian territory in modern day Syria so they wouldn't have access to reliable supplies from a friendly (or at least co-operative) population.

20,000 troops equals 40,000lbs of bread, 3000lbs of meat, and 30,000L of drink per day. Plus fodder for the chariot horses and pack animals, plus probably an equal number of non-combatants who were also attached to the army.

Some of that could likely be made up via foraging and looting, but I find it highly unlikely that there would even be enough calories available in the vicinity of an army like that to actually keep them fed.

This speaks to a big problem with Wikipedia as a source. If you actually look at the citations for both the size of the Assyrian army quoted above as well as the size of the Egyptian force at Kadesh you'll see that in both instances they cite a book by Osprey publishing. Osprey is a great publisher in a lot of ways, but they aren't academic. Their primary market is wargamers and modelers. In fact, both books cited were written by a fellow by the name of Mark Healy. Not to disparage Mr. Healy but he has a masters degree in Political Theology. Not History or Near Eastern studies or Archeology or anything related to the field. He's a hobbyist, not an expert.

As a matter of fact, the 20,000 number looks like it is actually citing a website article from ancienthistorylists.com with an article titled "Top 14 decisive ancient battles"

The thing is that even primary sources from the period are likely unreliable. Between propaganda, translation issues, and transcription issues it is impossible to say with certainty how many men were actively under arms...

So, lets look at it from the other end. What is reasonable? Realistically it comes down to food. So the primary source of calories for all of those societies would have been bread. A 1lb loaf of whole wheat bread has about 1100 calories in it. An adult male needs between 2000 and 3000 calories/day. We'll assume a goal of around 2500 calories/day as necessary to keep your soldiers capable of actually fighting. Most of that will come from bread and it wouldn't be surprising if each soldier would have eaten about pounds of bread per day which would net them 2200 calories.

Also, I'm fully aware that Ramesses did not have access to modern food scientists, but the reality of it is, that he would have been aware of the fact that a starving army will not perform well, and as armies throughout history have discovered fucking around with food very quickly leads to finding out that bad things happen to said army.

Now, man cannot live on bread alone. Especially if you want them fit to fight. An oz of red meat has about 75 calories. we're short about 300 calories so that works out to about 4oz of red meat. You could probably cut this back a bit if you supplemented with other things, particularly veg but you also need to remember that unlike bread, meat spoils very quickly. This means salting or other pre refrigeration methods of food preservation or keeping your meat on the hoof for as long as possible which will in turn create a massive logistics train.

Also keeping in mind you need to provide your soldiers with something potable to drink and this likely meant watered down wine or beer. Again assuming reasonably strenuous physical activity in a hot climate like the fertile crescent you are looking at about 1.5 liters of water per day.

Now is when we break out the calculator. If you are fielding 1000 men this means that in order to keep them from starving you will need to provide 2000 loaves of bread plus about 150 lbs of meat and 1500 liters of drinkable water per day.

Now, if you are fielding 300,000 men... well you are looking at 600,000 loaves of bread, 75,000 lbs of meat, plus 450,000 liters of drinkable something.

That's a lot of food for a modern army to manage with shelf stable MRE's (only country in the world I would bank on being able to pull it off is the USA) nevermind the 'logistics' system of a pre-20th century army.

*Edit* fixed the numbers for meat requirements.