Most of economy is like this nowadays.
“We’re used to the monetized form of this system, where the state pays those non-subsistence laborers, who can then buy their subsistence needs from the broader civilian economy (the loop then generally being completed with those civilians use that money to pay their taxes). But as we’ll see, that is not the only way to meet these costs and indeed not necessarily even the most common way. So we want to think about this, in its most simplified form as a question about how we move food (and other stuff, including workers!) from the agricultural economy to military purposes which have no real economic value of their own (you can’t eat a fort). That’s our problem.”