Every majour empire has ended in basically the same way. The costs of upholding the empire has increased over time (standing armies, bureaucratic control, areas of influence and so forth), until it finally gets too large, and the empire strangles itself. As this process goes on, the past belief amongst the inhabitants of the countries the empire occupies/controls that the empire is «untouchable», begin to deteriorate. Public uprisings get more and more substantial, until it gets out of hand, from the empire’s point of view. This played an important role in for instance the British Empire’s demise, and we see it building now against the US’ Empire.
The US economy is very fragile, and the US know it. They are in an unwinnable fight to control their economic strength by controlling resources across the world, especially in the Middle-East. In maintaining this control, the costs of the empire are increasing, which will ultimately lead to the end of the US global hegemony and empire.
The process is still in its early phase, but we see signs of the US empire deteriorating. It is vital that oil are traded in dollars (here Iran enters the picture), and the «virtual senate» plays an important role in forming policies in far-away places. If they lose these mechanisms of control, the empire will soon dwindle into a downward spiral.
It’s been estimated by several people (for instance Johan Galtung) that in 15-20 years’ time, the US empire will be over. Unfortunetely a new one will probably take over. So far the contenders are the EU and China. Both are much better than the US (at least in their current forms), but as their strength increases, they will likely try to copy the actions of the US (and former empires), and the process will duplicate itself.
The alternative, of course, is real democracy in the sense that the people decide the policies, or at least have a heavy influence on policy decisions. If that happens, I believe there will be a future for mankind. If not, we will either destroy the world by consumption and pollution, or we’ll blow ourselves up in a global war using whatever means necessary, including nuclear weapons. It is also possible the US’ demise will include the use of nuclear weapons. It depends who are in charge in the US at the time, and how the demise «go down». The Soviet Union crumbled relatively peacefully. We can always hope that the same will be true of the US’ demise.