Money and blood

When Youngest Son was in Civil Air Patrol, I hung out with some of the adults of the group for a while. One of them asked, "What keeps a plane in the air?" I figured there was a trick to the question, and there was. He went on: "Money."

Yes. That. That's what keeps a military running too, and has done so at least since the Crusades. I'm assured that amateurs discuss tactics and strategy, and professionals discuss logistics. That makes sense; if your men and stuff aren't where they need to be or when they need to be, or are stepping on each other, your tactics don't matter much.

Your logistics demand money.

It isn't just a matter of making the materials you need for war. (We already have people whose job is to worry about strategic materials and strategic industries.)

The money that keeps the military going comes from taxes, and if businesses are closing because the war cut off imports or too many workers got drafted, some of that money dries up. A command economy doesn't work terribly well for very long, even in wartime. Managers aren't that smart.

We've tried to trade money for blood. We don't just train our fighters, we equip them with gadgets to make them more effective, and provide expensive coordination tools (close air support, etc). Fewer soldiers do the work of more--at least in some battlefields.

Less money => more lives lost.

You need a strong economy to pay for readiness, and to keep the number of gold stars small in war. I assume we have people dedicated to figuring out how to degrade an enemy's economy--for just that reason. What do we call people who worry about our own economy's effect on the military? We do have them, don't we?

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Related considerations apply when you're talking about non-human enemies.