Guns, Germs, and Steel: The Fates of Human Societies and the recently-released Collapse: How Societies Choose to Fail or Succeed, had an op-ed piece in the New York Times noting some of the conditions that lead to the fall of various civilizations. Five major factors are the damage that people have inflicted on their environment, climate change, enemies, changes in friendly trading partners, and the society's political, economic and social responses to these shifts.
I think it goes without saying that the United States is inflicting damage on the environment. We are seeing unprecedented climate change as a result of that damage and damage elsewhere in the world. Much of the world is opposed to our policies of unrestrained capitalism and exploitation, and many are willing to attack us when the opportunity presents itself. Traditional trading partners in Europe are increasingly turning to emerging markets elsewhere and the dollar is losing primacy to the euro. Meanwhile, at home, the rich get richer and the rulers put their fingers in their ears and shout, "America is great!"
It might not be long. Either democracy will reform the country and correct the imbalance of power, or this may be remembered as the decade the giant fell.