This is one of the key reasons that I dislike a military/military SF novel: the military wouldn’t work that way.
I can buy battleships in space. I can buy WWIII. I can even buy magic and guns.
What I can’t buy is a lack of knowledge of basic logistics.
It takes a LOT of stuff to fight a war, folks, and the LEAST part of it is ammunition. You need food. Even more, you need water. Your troops can survive without food for a few weeks, but will die without water in a matter of days.
To move that, you need fuel. You need transports. You need someone to drive those transports and troops to protect them. Those troops need food, and water, and that food and water needs to be transported.
Sure, you could loot, for a time. But what can you loot in the middle of a deep forest? Desert? Ocean? And before you suggest hunting, consider how your army will scare away all the animals nearby. Armies make lots of noise. So do guns when hunting. And if you’re sending out troops to hunt, what will be hunting them? Insurgents? Monsters? Their own, very frightened, comrades seeing someone moving in the dark and opening fire? For some forces, in some wars, friendly fire has been the biggest driver of casualties.
You don’t have to solve these problems, especially if you’re writing SF. But you do need to acknowledge them somehow.
Mike Shepherd’s first Kris Longknife book solves this by making the hero a supply officer (of sorts) and showing all the troubles with logistics. It’s great, read it if you haven’t.
Heinlein’s Starship Troopers (the novel, not the movie) solves the problem by having a small, extremely powerful force overwhelm their enemies in a matter of days, or even hours. They don’t need logistics because the win (or lose) so quickly. Even then, Heinlein makes sure to acknowledge the fact that the Mobile Infantry has a limited amount of bombs, rockets, and nuclear warheads.
Or you can go the Napoleonic way, and have the marching columns eat everything in their paths. Napoleon instituted a standing order, for this exact situation: infantry plunders and loots on one side of the road, cavalry on the other. Just make sure that there are reasonable settlements and things to plunder, or you’ll have to explain how your hero has managed to find twenty fat cattle to roast for his 800-man company in the middle of the mountain glaciers…
In case you’re wondering why Poland’s gotten invaded so many times, add one part flat lands easy to march, or drive, over, to one part good farmland and relatively well-fed villages. Instant supplies for any attacker.
This isn’t rocket science, people. It’s the military equivalent of going to the supermarket. Everyone needs to do it.
And if you need to know what happens when you don’t, take a look at this Wendover documenary on why the Russian Army came to a stop in the current Ukraine war:
