How ancient armies, such as the Roman Legions, able to feed themselves during wartime campaigns?
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Rome had an AMAZING transportation & logistics infrastructure, honestly it would not be rivaled for a thousand years. Rome had specific administrative positions that would ensure grain made it from Africa & Egypt (plus Gaul, Sicily, and a handful of other productive areas) to the soldiers on the Germanic & Danube limes (the Eastern armies in Syria were much closer and easier supplied.)
Rome had a hub and spoke system, where a hubs like Cologone, Trier, Lugdunum, etc… operated as a grain warehouses which then distributed the grain down the spokes to smaller graineries (very similar to how fed-ex works today.) If there was a shortfall at one hub grain was pulled from others to even it out. This was the same for salt, olive oil, and some wines (other wines were sold by travelling merchants.) In fact we get salary today from the Roman word for salt because it was part of their payment. Anyway other food items (ones that were perishable) like meats, cheeses, etc… were acquired from local sources usually on cash payment, of course sometimes they were seized. Now this applied to Roman soldiers inside the borders of the Empire. Once they went on campaign things changed.
Caesar speaks of requisitioning grain (corn as they called it) from allied Gauls during his entire Gallic war, in fact ensuring those supplies was the job of one of his tribunes. There were times, such as sieges or marches through the desert, when rations became unavailable and soldiers just dealt with it. In one story in the Civil wars, Pompey had isolated Caesars troops and they were forced to make bread out of roots they scrounged (they threw the loaves at Pompeys soldiers who thought they were beasts for being able to maintain in such conditions) there is another story of Justinians troops having to eat the leather from their saddles and other gear because no food was available and the supply line was cut, but these was only because the supply line was interrupted.
When Roman legions would leave Roman territory and enter say Persia or Dacia they would still have secured supply lines, they would even detach cohorts to maintain these supply lines. In addition to that Romans practiced full scale pillaging of the land, this served two purposes, to give them food and to deny food to their enemies.
Rome did not have the massive baggage trains full of camp followers that we saw in the middle ages, but they definitely had slaves to help setup camp, cook, etc… Additionally there were always merchants (mercatores in latin) that followed the soldiers and sold a variety of goods, later emperors chastised their troops for wasting their pay of frivolities and luxuries the mercatores offered them (and usually for ridiculously high prices.)
and as a sidenote think about just how much grain was needed for a 50k army. If each man only ate 2kg of grain a day (which is what most think their ration was) you are talking 100,000 kg (or 50 tons) of grain needing to be delivered each day, that kind of logistics 2000 years ago boggles the imagination.
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