In close-in battlefield combat when bullets are flying, soldiers stop thinking with the forebrain (that portion of the brain which makes us human) and start thinking with the midbrain (the primitive portion of our brain which is indistinguishable from that of an animal). Animals viciously attack other species, but they resist killing their own kind. In conflict situations this primitive, midbrain processing can be observed in the powerful resistance of humans to kill other humans.
General Marshall, Official U.S. Historian of the European Theater of Operations in World War II, did post-combat interviews and found that that only 15 to 20 percent of the individual riflemen in World War II fired their weapons at an exposed enemy soldier. Crew-served weapons, such as machine guns, were almost always fired and firing greatly increased if a nearby leader demanded that the soldier fire. Other similar studies have shown that, when on their own, most individual combatants throughout history seem to have been unwilling to kill.
The US Army accepted Marshall’s conclusions and, instead of having trainees fire at bull's-eye targets, they used realistic, man-shaped, pop-up targets that fall when hit to help condition the men to killing. This type of conditioning is the only technique which will reliably influence the primitive, midbrain processing of a frightened human. Conditioning techniques increased the rate of fire to approximately 55 percent in Korea and to about 95 percent in Vietnam.