We all know ‘an army marches on its stomach,’ but it’s not like Napoleon discovered something new. Vegetius (De re militari) and Sun Tzu (The Art of War) were well aware of this concept, as was Alexander the Great (Engels, Alexander the Great and the Logistics of the Macedonian Army, 1980). And it wasn’t news to them, either. Pre-modern military commanders knew this; they planned for this. They paid attention to logistics.
Fantasy writers should, too.