Good question. A founding principle of the US, I'd say it's part of the problem. Affluent countries populations start to expect a certain level of ability to pursue it, and expectations seem to reach a point where they begin to exceed economic growth. At this point, affluent nations are forced to begin subduing lesser nations in order to even just maintain the status quo. No super power, throughout history, has ever been able to maintain its position at the top of the tree without making sure those beneath it know their place.