If it’s one thing we can learn from history, it’s than no empire is forever. Rome, Ottoman, England, Germany, France… All controlled or influenced a sizeable chunk outside their own immediate country at one time and doesn’t anymore. The UK is still a major player in world politics and economics, but far from the power it was up to 1914. Italy is also strong, but in no way as influential as Rome was.
Since WWII, the single biggest power has been the U.S. USSR tried to compete, and while they could do it militarily, their cultural and economical influence during the cold war was very much smaller than the U.S. For a period during the 70’s to mid 90’s, it looked as if Japan could be a contender, but not any more. The EU is now a bigger consumer market than the U.S. but the military has still a long way to go.
China is moving fast and will probably be a very big influence during this century. I doubt it will come to an open conflict with the U.S. any time soon, but I’m sure there will be tension and smaller “trade wars”.
Now, maybe the U.S. will still have as influential a role in 500 years as it does now or maybe not. I don’t think it will, but that’s just an opinion of course. So if the U.S. would slide back to being a big power, like the UK or China, but not the dominant world power it is now - how would that happen? Why? When? Would the U.S. be replaced with another power?
(I hope we can keep out some of the American partisanship out of this. I dislike and distrust GWB, but I don’t think it’s possible to revert the inertia of a country as big as the U.S. in eight years. Also, this OP was inspired by an article in the summer issue of the Atlantic. It’s only available to subscribers now, unfortunately.)