The Dark Forest Theory of the Internet
A bit off-topic, but I've kind of struggled mentally since finishing The Dark Forest.  Even though it's science fiction, it actually seems hard to argue with the theory in the book -- that civilizations must act to eliminate each other or they are overwhelmingly likely to be eliminated themselves.  I'd like to believe it's not true, but so long as any two civilizations are likely to have dramatically different rates of technological advancement and so long as crossing the gaps in space between civilizations takes sufficiently long due to the laws of physics, it seems hard to deny that there might be strong reasons for civilizations to fear each other.