Good article by Niall Ferguson in the Review section of the Fin last week (15 Dec). He talks about the limits of imperial power and how more recent empires (eg Russian, German) are lasting less time than previous ones (Roman, Ottoman, etc). He makes the point that empires emerge as world powers because of the economies of scale they make possible. They can build bigger armies, tax more subjects, provide more public goods. They endure so long as the benefits of exerting power over others exceed the costs of doing so and that resistance is outweighed by the benefits of being dominated.
He cites three reasons why the current US imperial ambitions will be short-lived. These are troop deficits (which has long been a sore point for military commanders on the ground), the budget deficit (which is reducing the capacity to fund Iraqi reconstruction), and the American attention deficit with the lack of public support for this foreign war.
This reminds me of the excellent polemic book by Zia Sardar and Merryl Wynn Davies American Dream: Global Nightmare. The book explores the mythology of America and finds that Americans believe that they have a right to be imperial, that cinema is the engine of this empire with celebrity its common currency and that war is a necessity. To combat this mythology, the authors state that America must rejoin the human community through dispelling the notion that America is the lone conscience of the world and then join human history through self-reflection on the perils of what happened to the Roman empire that had a powerful Executive, an irrelevant legislature and a content and distracted populace.
Of course, this does not mean that empires are history. The Ferguson article concludes with the premise that as population increases and natural resources (like oil and water) become more scarce, imperialism can easily resurface. The Chinese links to resource rich countries is just one example of this resurfacing.