You may have seen this Mother Jones article on the Pentagon study of historical precedents for the supposed "American empire."
Don't Know Much About History
A nice, silly contract for those five authors. It may make amusing reading for the brass hats, but the smart ones (not all generals are uncultured -- Petraeus actually appears well read) will pay little attention. The U.S.'s global reach is actually of a very different order than any of the four supposed precedents, and each of them was fundamentally different from the others. The Roman empire is the only one of the four that offers any useful comparisons, but the likeness is also deceptive. I'll have to give this more thought, but here's a first impression: Rome extended its power from Rome itself, over many centuries, first conquering its immediate hinterland in central Italy and subjugating Etruscans et alii, then extending its frontiers until it ran into other empires to the south and east (especially Carthage) and more "barbarous" tribes west (Gaul, Hispania, Brittania) and north (Germania et al.), and Rome's internal transformations over those centuries including numerous civil wars, assassinations and other treacheries.
U.S. expansion has developed in the opposite way. Yes, the Eastern Seaboard states progressively conquered territories on their frontiers (e.g., in the war with Mexico and the "Indian" wars). But the larger, world-wide expansion of U.S. power was not generated mainly from Washington or New York or any other U.S. city, the way Roman power was generated from Rome. Instead, those other parts of the world have transformed the U.S. and created those cities (NY, LA especially) as foci for further transformation. Globalization, the collective force of people all over the planet (those huge immigrant streams, and then movements of capital), has produced many foci for concentrating and emitting its force (Sao Paulo, Buenos Aires, Montreal, Quebec, Sydney, Johannesburg and others), places that didn't even exist before the modern (post-Columbus) era, each competing with all and all of them stimulating ever faster, denser relations of global power. The U.S. is the supreme product of globalization, not its original author.
This is just a rough statement of an idea I want to explore. My strong hunch is that the radically different origins and structure of U.S. power (compared to Rome and any other predecessor) foretells a radically different evolution and, ultimately, dispersion of that power. Dispersion rather than collapse is what I foresee.