This is not true at all. Supply lines have existed as long as organised armies have existed. Many major military campaigns have made use of supply depots, supply ports, supply trains (on pulled wagons, long before what we now call trains existed), etc. Napoleon also made use of them. Only, it's very expensive, and it's a lot cheaper to live off the land. Only living off the land also takes time, so if you want to move fast, arranging your supplies up front allows your army to move faster. But long supply lines deep into enemy territory are incredibly expensive, as well as vulnerable to raids. Which means you need to protect them, which makes them even more expensive.
It was not logistically impossible, it was incredibly expensive. And more so the deeper into enemy territory you get. But plenty of armies throughout history did bring their own supplies.
> Rather than reinvent the wheel here, I’ll note that K. Chase (op. cit.) ran these numbers assuming two-horse 1400lb wagons and found that assuming the army acquired no local food (but could get grass for the horses), for a group of thirty infantryman the first wagon doubles their range from 120 to 240 miles (less really, horses cannot be worked so many days consecutively). Doubling again to around 400 (accounting for horse rest time) requires not two but six wagons for thirty men. To double the range again would require more wagons than men.
By around 3 months of campaign, you need more wagons than men to provide a supply train that doesn't require local forage for your army (but require it for your animals). Supply trains in pre-modern times are less "let's provide all the food for our army on campaign" and more "provide the mobile granary to store everything we're looting for a few days before it's eaten." In premodern times, absent the possibility of naval supply, not foraging the local land for your food is logistically impossible.
I never said it's a good idea for all situations, but for some. It was most definitely done, so the claim that it was impossible is clearly nonsense. It was impossible in some circumstances, but possible in others. And of course it's a good idea to use local resources when they're available, but there's a big difference between that and relying entirely on local resources.
Armies used supply lines over land during the 100 Years War[0], Romans maintained a network of supply depots[1]. It happened during most time periods.
Just like there's a limit to how much food you can move how far, there's also a limit to how large an army can live off the land. Beyond a certain size, you simply need to bring your own supplies. In fact, one of Napoleon's innovations was that he organised his armies in a way that allowed larger armies than before to live off the land. But larger armies did exist before; they just had to bring supplies.
And of course you're not going to supply a large army over a distance of hundreds of miles using just carts. They used ships where possible, they used supply depots, and organised the most efficient routes by which to maintain those. But you bet lots of land travel was involved as well.
It's absolutely true, and the only thing I missed and you are not wrong about is that supplying armies via ship was possible, if they only operated near ports. That's exactly why the Roman empire conquered lands around the Mediterranean but never advanced deeply into central Europe or the middle East.
But no, neither depots nor wagon trains can supply a large army over long distances. It's not too expensive, it's impossible, because the animals that draw the wagons (and build the depots) themselves eventually consume more supplies than they can draw.
A force of 2000 men over 100 miles? Yeah, that may be in "doable, but expensive" territory. 50,000 men over 500 miles? Forget it.
They did conquer the entirety of Gaul, which is not just coast. And France isn't small. Romans built roads, supply depots, carried supplies on pack animals.
And if you think you can move 50,000 soldiers 500 miles deep into enemy territory without supply lines, you can forget about that too. Napoleon did use supply lines from Poland into Russia. Remember: we're talking about Sun Tzu's advice not to bring your own food but take it all from the enemy land, and I'm saying that doesn't work in all circumstances. I'm not saying it never works. But there are very good reasons why historically, large armies did rely on supply lines.
It was not logistically impossible, it was incredibly expensive. And more so the deeper into enemy territory you get. But plenty of armies throughout history did bring their own supplies.