Lower class = Military might?

So in Norman Mailer's The Naked and the Dead, General Cummings is a pretty insightful fellow.

"Well, then, follow me out in this. And you're going to have to take my word, for I've made a study. When I was your age, a little older, the type of thing that preoccupied me was what makes a nation fight well."
"I imagine it would be a king of identity between the people and the country whether it's for good reasons or bad."
The General shook his head. "That's a liberal historian's attitude. You'd be surprised what a tiny factor that is." The lamp was beginning to sputter and he reached over to adjust the valve, his face lit rather dramatically for a moment by the light source beneath his chin. "There are just two main elements. A nation fights well in proportion to the amount of men and materials it has. And the other equation is that the individual soldier in that army is a more effective soldier the poorer his standard of living has been in the past."

Neat.