...that the US empire is more similar to the Spanish, there are a few major differences that might make the outcome quite different.
Spain's "wealth" did indeed come from its empire and the creation of a rentier class (as the US is turning into, or at least would if GOP/DLC neo-lib economic policies continue), perhaps a greater source of wealth for the US is the dollar.
Owning and printing the dollar, the "moneta franca" of the world, creates an awful lot of wealth out of thin air. More aspects of our wealth are also illusory; the GDP would probably be at the EU level if it wasn't for energy consumption - which is far larger because of the long distances that transport entails in spread-out US. The same goes for health spending - we pay far more than anyone else for far less services, artificially increasing our apparent wealth.
This COULD be a recipe for disaster, as it was for Spain. The Spanish ducat was the "moneta franca" for awhile, but it was a currency based on its actual metal content. Our money is fiat, especially after Nixon took us out of Bretton Woods and the gold standard - and it is the basis of most countries' foreign currency holdings. The fact that so much of our debt is in the hands of foreigners means that the rest of the world has a vested interest in maintaining the illusion of American wealth and the apparent strength of the dollar --- to abandon the buch would mean abandoning a significant portion of their OWN wealth.
Yet, as you well note, our situation is particularly unstable - because it relies on "market forces" which do not necessarily follow the dictates of logic. The fall of the paper tiger economies nearly brought the whole globalization aparatus to the ground in ruins - and a run on the dollar could mean disaster.
Yet what is the government in a cleptocratic atmosphere as ours is today? It is merely a tool, a vehicle, for corporate interests. And these interests are multinational, with absolutely no sense of patriotism or of responsibility. We have seen how they are quick enough to transfer operations abroad, put money in fiscal paradises...
Unless the sh¡t hits the fan, I figure that the decline of the American empire will be a gradual affair. Money will escape as corporations move abroad, leaving the few rentiers well off and the rest of us screwed. The avalanche might occur when the gubmint can no longer pay into the military/industrial complex...
But HOPEFULLY the pendulum will swing beforehand.
Another frightening possibility is the idea of a REAL terror war, not by a few Muslim lunatics but by what is known as "the South" (Africa, L. America, etc.). This
http://www.democraticunderground.com/discuss/duboard.php?az=show_mesg&forum=132&topic_id=2701091&mesg_id=2701091 shows that our greed has no limits - I wonder if the patience of the 3rd world does.