And yet, they did anyway. Why was it that the English were able to move thousands of men around the French countryside for months? Either French border fortifications weren't doing the job of stopping their supply caravans or the armies were able to subsist off local area forage. How were the frequent large-scale raids of the Iberian peninsula conducted? How did first the Arabs then the Seljuk Turks remain such a persistent threat to the interior of Eastern Roman Anatolia, even in the face of unreduced, uninvested border fortifications?
Why didn't any of these armies go hungry and fall apart?
Your claim. I deny it.
This is pretty irrelevant the point being made. Taking your claim at face value -- "Armies could not advance without reducing fortifications in their way which prevented them from re-supply..." -- then the French should've needed zero tactical competence. Starvation should have done all the work necessary. But the English didn't collapse due to starvation, in spite of being in the French interior for months.
Sure. But that's not the solution people are proposing in this thread, that local area forage be a consideration. It is all various combinations of flypaper mechanics for castles on armies, designed explicitly to create a frontline when that sort of thing was firmly post-medieval.