I think non has it exactly backward. America is at it's weakest point geopolitically in several generations. The foundation of global dominance is and always will be built on the back of national GDP.
http://en.wikipedia.org/wiki/Historical_GDP_of_ChinaAmerica's relative proportion of global GDP has been shrinking since the 1970's. One cannot expect to remain the lone superpower in the world, as we once were, when another nation has a significantly higher GDP and population. Even at it's height the soviet union was no where near the economic size of the united states, which is why we ultimately won and Russia is still...Russia. Furthermore, as menial powers gain in size, population, and sovereignty, a powerful state cannot operate as it once did, even if it is the most powerful. Iran is a great example. From ww2-1970's we effectively bossed the country around with little effort. We invaded them, in a move so hilariously easy it's an afterthought in history, and ran a puppet government there with little trouble for decades. Now, Iran acting up is a centerpiece of US foreign policy, and invading them is out of the question because no one thinks it's militarily feasible.
