I've been reading a primatology book recently and came across a description of Chimpanzees going to war. This has really freaked me out for some reason, I think its the evolutionary implications that there exists a hard wiring in male brains to organize and kill other groups of the same species. I'm really not sure why I've been so disturbed by this.
Anyway, via FARK, I came across a Cracked article that deals with the topic:
http://www.cracked.com/article_18766_5-creepy-ways-animal-societies-are-organizing.html