Vertical vs Horizontal Aggression
One of my dreams last night went into a discussion of political history. Feudalism is born as warring groups of all sizes develop structured alliances to defend against "horizontal" aggression by other groups. But soon enough, the pressure is towards the information and action focusing on "vertical" aggression between lords and vassals: you're in contact with them, you know them more and more intimately, and inasmuch as feudalism is effective, it does raise the costs and risks of horizontal aggression. This pressure has two effects: One, horizontal aggression diminishes at all levels but the top levels where it is concentrated: kings, dukes, earls, whoever has effective as opposed to nominal sovereignty. Two, power gets concentrated as the intermediate levels slowly get ground away from above and below through vertical aggression while the top levels grow through horizontal aggression, leaving centralized states as an end-game. The theory makes measurable predictions, that indeed horizontal aggression diminishes with time while vertical aggression increases; it doesn't actually stop at feudalism. And it was a pretty cool dream including diagram with fat arrows of horizontal inward and vertical outward pressure squeezing the aggressive action.