Maybe. But I tend to think that what will happen is that it's time for some other super power to rise (maybe China). Every dog has its day. The U.S. will probably become like Great Britain (it was said in the 19th century that "the sun [did not set] on the British empire"). So I don't think the U.S. will become a third world country. But it will not be on top of the world anymore. There will be a shake-up. Life will probably change drastically. There might be some chaos and some scarcity for while. And that might not be such a bad thing. Nothing lasts forever. And nothing ever stays the same.
What I do tend to think about more is what we will all do when the oil runs out (I believe in peak oil)? That is a global problem and I'm yet to hear any plausible solutions. I read somewhere that we need to stop relying on fuel period, i.e. that we shouldn't be looking for alternative energy sources because beyond the sun and our own strength/will, there is no other fully renewable, 100% sustainable energy source. Even bio-fuel is just not sustainable because corn actually depletes the soil of nitrogen. (In fact, traditionally, corn was never planted alone so imagine how destructive it is to our eco-system all the mono-crops of corn that is grown and subsidized by the U.S. government.)
Can you imagine life without fuel? No oil to drive your car. No oil to power agri-business (so you'd have to plant and harvest your own food using your own energy and ingenuity)? No gas to light your stove or your spliff for that matter? No electricity to power your washing machine and dryer? Your stand mixer??? Your power tools? Talk about going back in time. It's daunting to me. Yet intriguing at the same time. I think I might actually enjoy some aspects of simple, back to the land living. I think I might enjoy what we come up with to power things in the future (hand cranks, water power). I know that the Earth would really appreciate it.
I wonder what the next step for humanity is. What an interesting time to be alive!