When I talk to a class and all the students listen without saying anything, I figure that they are agreeing with me.  When someone makes racist or other bigoted comments in my presence and I say nothing, he is going to assume that I agree with those opinions, or at least that I don’t mind them.  The Nazis were never the majority, but the silence of so many of their neighbors allowed them to think and act as though they were.

The most effective way to reduce bullying behavior is for bystanders/witnesses to speak up in support of the person who was the target of the aggression.  (Note: That is very different from verbally attacking the “bully”, which is adding to the aggressive behavior.)