Ah yes, pottery. So durable and capable of being used to transport large volumes of water over rocky terrain and primitive roads without breaking, very plausible. And at any large volume (over 500L) wineskins would leak a ton.

Did mesopotamian cultures have the technology to create large watertight containers?

100% would need more. Soldiers are marching, building camps, hauling gear, etc. Hard work, and tiring, would need at least 4 L per day and 3000+ calories imo, unless you want your army to be emaciated and exhausted in a couple of weeks.

Great points. Also, I think 30,000L is way too low. They are soldiers in Syria. Easily 4 liters per person per day. Moving 80,000L of water per day in animal hide skins using horses and people is a crazy logistical feat over any distance more than a couple miles. They didn't have water containers on wheels, right?

You realize that the logistics and technology to feed and coordinate that many people didn't exist until modern times, right? 300K men means 2.1 Million gallons of water and 4.2 million pounds of food per week. You realize this is an insane amount of material to produce, store, and ship solely with man and horsepower, right?

Just doing napkin math: A Typical horse can walk 50 miles a day and carry 300 lbs. Assuming the military campaign is only 25 miles away from the city that has 10's of millions of pounds of food stored for war, you would need 5714 horses working 24/7 just to move the food and water to the soldiers. For every 25 miles farther, you need 5714 more horses. Also, these horses need food+water, gear and weapons need to be moved. So if you wanted to wage a war 100 miles away from your city, you needed more transport horses than all of the legions of the roman empire at peak power (22k or so cavalry).

Just crazy to think about, and then add that you need to feed these horses, and that you need to feed the cavalry horses as well. Don't know how much a horse eats, but i imagine its a lot.