Below I have a quote from an article which can be found here -  http://www.commondreams.org/views03/1210-08.htm

Repeat after me: Kill Kill Kill

Is that murder going to be convicted of a crime - doubt it, it puts trained killers off when they think they may have to take responsibility for their actions.

"Perhaps one of the most compelling arguments against sending our young women and men to war is that most of us don't want to kill at all. In spite of being taught how glorious the battles might be, most of us don't comply with the request to kill. In his fascinating book On Killing, psychologist Lt. Colonel Dave Grossman devotes a whole chapter to the "Nonfirers Throughout History." Research has found that throughout history, in any war, only 15% to 20% of the soldiers are willing to kill. This low percentage is universal and applies to soldiers from every country throughout recorded history. Interestingly, even distance from the enemy does not necessarily encourage killing. Grossman offers the fascinating finding that "Even with this advantage, only 1 percent of the U.S. fighter pilots accounted for 40% of all enemy pilots shot down during WWII; the majority didn't shoot anyone down or even try to."

The U.S. obviously didn't appreciate this low percentage of non-killers, so it began changing the way it trained its military. Americans began using a combination of the "operant conditioning" of I.P. Pavlov and B.F. Skinner in their training, which desensitized our soldiers through repetition. One marine told me that in basic training not only do you "practice" killing incessantly but you are required to say the word "kill" in response to virtually every order. "Basically the soldier has rehearsed the process so many times," said Grossman, "that when he does kill in combat he is able to, at one level, deny to himself that he is actually killing another human being." By the Korean War 55% of U.S. soldiers were able to kill and by Vietnam an astounding 95% were able to do so. Grossman also states that Vietnam is now known as being the first pharmaceutical war in which the U.S. military fed our soldiers enormous amounts of drugs to dull their senses while they engaged in violent behavior and they are likely doing the same in Iraq."