anyway.

, let me give you the pre-Columbian Maya. Mayans had no horses, no wheels, no boats (their rivers and coasts were in the wrong places relative to where they were trying to go, anyway), so a Mayan army consisted of lots of guys on foot, eating corn (maize) carried by... lots more guys on foot. Each guy on foot carrying corn is also eating some of the corn he's carrying—and when he runs low, he has to go back to the corn depot for more corn—but he has to go back before he runs out altogether, because he has to eat something on the way back! Immediate result: The "fuel fraction" of Mayan logistics was appalling; their supply chain (of people) tended to eat more corn than it delivered to the army. Midterm result: Mayan armies couldn't march more than a few days' beyond their home bases. Longterm result: Mayan kingdoms remained small, often so small you could see the temple-pyramid of one kingdom from atop the pyramid of another. Ultimate result: Mayan civilization never achieved the political unity that might have helped it manage its other problems (e.g. overpopulation relative to their water supply and arable land) and collapsed.

Stupid corn.

You can run this in reverse, too. Your fuel fraction gets a lot better if you have, say, carts with wheels drawn by oxen (horse-drawn carts not so much in the ancient world, when horses were rare, scarce, and reserved for riding). But you can overdo it, because an ox has to eat too. Ancient Greek city-states were militias of well-to-do citizens able to afford their own armor. This made for tremendous political cohesion, but all these proud citizen-soldiers didn't want to schlep their own stuff to war, nor to pay taxes for bureaucrats to organize supplies, so a Greek army went to war encumbered by a tremendous number of carts carrying everyone's armor, spare blankets, extra wine, etc. etc., plus more carts hired by local merchants selling food to the army in the absence of a formal supply system. This hurt their "fuel fraction," reducing range, and clogged up the narrow roads, slowing their march and reducing range further. Result (abetted by narrow mountain passes): Lots of independent little city-states.

Then came Phillip of Macedon, a barbarian autocrat whose soldier were subjects, not citizens, who borrowed all the best parts of Greek military science and got rid of all the hangers-on with their extra oxcarts. Immediate result: His armies had a better fuel fraction, increasing range, and marched faster, increasing range still further. Midterm result: The conquest of Greece. Longterm result: Alexander the Great. Ultimate result: Western civilization as we know it.

The Romans underwent the same transformation on their own steam, turning their citizen militia into a professional army where everyone humped his own gear (the great reformer Marius took away all the carts but the essential ones and gave troops a walking-stick instead, called, bitterly, "Marius's Mule").

Fast-forward a few hundred years—the Mongols: They moved so fast and appeared in so many places in short succession their victims thought they'd been hit by massive hordes. In fact, "horde" comes from the Mongol "ordo, "light, easily transportable tent used to camp in the field," and the Mongols were usually outnumbered. Their horses weren't even particularly fast. But the average Mongol rider had ten or twenty scrawny step ponies on a string, so he could change mounts as soon as one got tired and keep riding.

Fast forward—Napoleon: Before the French Revolution, European armies were made up of conscripts, convicts, and assorted scum, and no one trusted them out of their officers' sight, so they marched ponderlously, in large formations, from one supply depot to the next. Then the French got all excited about liberty, equality, fraternity, and global domination (like some superpowers we know), and discovered that they could send their newly patriotic troops out to steal chickens from every peasant in the vicinity and still see them in the ranks again that afternoon. Turned loose on other people's territory, the French could dispense with supply wagons, rely on "foraging" (i.e. theft), and advance with staggering speed—until they invaded Russia and discovered there's nothing to steal there in the winter, especially once the Russians burn their own villages.

A modern, high-tech, highly mechanized army lives out a dangerous paradox with its fuel fraction, because its fuel fraction is, literally, fuel: Instead of Mayan porters or Greek oxen hauling loads of food and eating some themselves, a modern army has trucks (and planes, and ships) hauling fuel and burning some itself. The fuel fraction of gas-burning vehicles is way better than that of food-eating animals...as long as you have gas. The minute you don't, you suddenly find yourself in a Napoleon-in-Russia situation: You may be in the best, most fertile land around, but if you ain't got gas, your army is effectively starving.

Historically, by the way, the best way around the whole fuel-fraction problem is ships. If they're wind-powered sailing ships, of course, they don't have any "fuel fraction" problem at all, because they don't have to carry their own energy source; but even coal-burning or gas-burning ships have a much better fuel-fraction ratio than any land vehicle, because they're moving through a medium with vastly less friction and can thus move more mass with less energy. This is a big part of why the Roman Empire ended up all around the Mediterranean but, in most places, not that far inland; why the British Empire ended up in India (but not Afghanistan) and Hong Kong (but not central China); and why the US military still stockpiles ready-to-go equipment in "prepositioning ships" near crisis zones.