This is an article from the NinZine . Admittedly, it seems contradictory that martial art training can make people more peaceful. But consider, if you will, that many times people fight and resort to extreme methods because of a lack of control.
Fear also causes many people to pull the trigger on a gun.
Martial methods give the practitioner control over himself and violent situations. This is not always the case with everyone, of course, as there are always bad teachers of just about anything out there, and bad students with misplaced motivations and or desires. The sincere practitioner who has a good teacher generally learns much self control over the years, and learns that with simple movements he can avoid attacks that normally terrify most people. In my own life, I found that the more of the martial way I learned, the less I would feel like reciprocating against someone who just flys off the handle for no particuliar reason. I now have the ability to look at someone's pain or hurt in a objective way, not afraid of a threat of violence.