Within the next generation or two, China will have the world's biggest economy. The US can do nothing about it, without cutting severely into the living standards of its own population. The American economy, as others have pointed out above, is a giant bubble that could burst at any time.

Meanwhile the US has discovered that for all its monstrous spending on "defence" (which embraces a huge spend on offensive capability) it has just found out that policing the world is not as easy as it had supposed. Right now it is struggling to make its writ run in a tiny part of one of the poorest nations on earth (Afghanistan) and in Iraq, which has been bombed to ruins by two wars.

For better or worse, the Romans introduced a degree of civilisation to the territories of their empire; likewise the British. The US, having destroyed Iraq, finds it has no capacity even to make good the damage it's done. Iraq now has less electricity than under the Saddam regime, and out of 2,300 construction projects identified and promised, 140 have been started. Bridges destroyed during the war have yet to be repaired, and water supply systems in the south of the country, similarly destroyed, are still not working. (Why it was necessary to destroy civilian water systems was never explained.)

Anyone who thinks so tenuous an empire could survive for centuries (even ignoring the fact that the empire is intent on destroying the global environment well within that timespan) is living in Cloud Cuckoo Land. Its days are numbered, and I just wish DougR could hang in long enough to count them down.