The model of 'decline relative to other powers' does not properly capture the dynamic at work.
The US is both pre-eminent in the world, and at the same time, paralysed by its own insecurities.
It is not the Taliban, the 'resistance' in Iraq, Ahmadinejad or Chavez that are causing the US to hesitate and falter - rather they are only taking advantage of America's loss of confidence.
There is no real challenge to US power from without, there is a collapse in the confidence of US power within.