There is a difference between a boxer who wins, and a boxer who continues to beat his opponent into a bloody pulp long after the bell has sounded. There is a difference between one who provokes a fight, and one who is provoked. Both are fighters, but that is not the issue. Both the one who is provoked, and the boxer who wins fairly and with respect, are not blamed for their behavior. But there is a point after which society condemns even the protector for his actions.
That is the line that the Zen trained martial artists does not cross. He truly acts only in response to aggression. He does not seek it out. When made, his responses are non-resistant and non-violent. He is a man of peace, content to paint his sumi-e, do his ikebana, and practice his swordsmanship, in all ways constantly refining his technique.
When called upon to act, he simply acts. When he paints, he paints. When he uses his sword, he simply uses his sword. When he is pushed, he does not push back. He lets whatever it is go right past him. His response is purely defensive. It is also decisive.
-From the introduction to The Book Of Five Rings