[PorterDuff]

"I remember seeing a program that claimed this is because soldiers today are trained to kill against their natural instinct. So traditionally only 20% of soldiers could actually kill, which would explain the low casualty rates in older battles pre-ww2."

That's a good observation, and not one that I thought of, but it does bring up the question of organized combat in the other 95% of human history. It would surprise me if a Roman got much choice in the matter from his friendly local Centurion.