Okay, I realize this is a huge, vague question. Please allow me to elaborate.

I have been thinking about the Holocaust and have had trouble wrapping my head around the sheer brutality committed by ordinary people. I simply cannot fathom how a society collectively does such things and seemingly ordinary people cooperate all the way.

Not understanding this scares me, because it makes me ask "What's to stop something like this from happening here and now?"

So, my question(s) are thus: what are the specific factors that allowed the Holocaust to happen where and when it did (Germany in the 30's), and why did it not instead happen in a different place at a different time? Why not, for example, in France in the 1880's? Why not in America now?

Most importantly, what's to stop it from happening again?

I realize this is a huge question that may not anserable by a single Reddit comment, but I will appreciate any answers, information, or book recommendations on this subject.