Typical "Pecking Order of Nation" and trying to make a parallalel with the fall of the Victorian empire (Anybody read Howard Bloom's "Lucifer Principle")
America isn't a "Nation" anymore. It's a harbor of Corporation. 90% of the global Fortune 100 is US based.
As long as this remain, American will remain strong. If Corporate America sells out, or another nation offers a better haven for Corporations, America won't have any reason to remain as powerful. However, no meme system in the world can offer an haven like America. And not for a long time. That includes existing Mega Corps but also incubating the next multi-billion company like the current superpowers like Micrsoft, Google Yahoo who didn't exists 30 years ago.