How groups start forming, building a boundary between themselves and the outside, with hostilities following.
Inspired by this Germany Must Perish! - Wikipedia which is a perfectly understandable response to Germany trying to wipe out other nations. Of course the Germans used the book to support their arguments that they were under attack.
The Germany Must Perish story is a perfect example, but you see the same dynamics going on with Fox News and in truth the organs of my own side like msNBC. You seize on the worst acts of your opponents, and make them central, more important than they actually are (and so with increased polarization they actually become more central, it's a sort of self-bootstrapping process, akin to hyperstition). I think somewhere I called this "mutually reinforcing death spirals" but can't find where.
I should say I can't condemn the author for his strong feelings about Germany, I have certainly felt the same way, although I didn't let those feelings turn into an actual ideology or action plan.
From a diatribe to some wingnuts:
Re: sides. Reality is not two-sided, it's very complex and every individual has their own unique set of beliefs and values. Nonetheless, despite this diversity, humanity seems awfully good at forming up into opposing teams that go about trying to kill or overpower each other.
Sociologists have a technical term for this: polarization. It's a rather abstract yet very powerful force, one that is somewhat independent of the wills of the individuals involved. The dynamics of polarization are very evident in the present-day US.