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Being able to outspend the other side is definitely a good indicator of winning odds. There is some friction of course, but it’s a very rough approximation of how much industrial might you can put into the field. And just having money isn’t enough either, you have to be able to spend/convert it into a military power in some way.

So it stands to reason that wars will be as expensive as possible. It’s pay to win to a degree.

They were also financed with a finite resource, or at least, one that grew at a much slower rate, governed by the laws of physics as they impacted the economic feasibility of gold mining. Rather than an indefinitely large supply of "money future generations will pay back."

That's a point, but frankly the wars were expensive because you have an organized, uniformed military that has to abide by UN regulations and the Geneva convention fighting people (guerilla units) that aren't held to any of those things. There's a cost to being legitimate and it's more than people would think.