[maxerize]

I don't see why only recent times are indicative of this. America had few outside enemies in e.g. the 1920s, and yet the inner conflicts (Teapot Dome, Steel and Railroad strikes) during those years were nothing like our recent ones.

[wangii]

Maybe I should put it this way: US already reached its peak status, and it's very difficult to even find a way or area to "grow". In 1920s, although US became an economic super power, there were still plenty of room to grow in the area of international political system.