I doubt that Iraq, like Vietnam before it, could bring you down.
Iraq is nowhere near what your Vietnam commitment was anyway.
I would be inclined towards something more insidious, like the oil running out and there being and no mass cheap credible alternative, that sort of thing.
Water (various civilisations)
Climate change(ditto)
disease/health(ditto)
pollution problems(something that China is heading towards fast)
Social change (Britain had to hand back her colonies which were 'nice little earners'.)
Food shortages
If the Global Corporate structure becomes as socially unacceptable as Colonisation did, the US would be severely affected.
It sounds silly, but so did handing India and Africa back to them 100 years before we actually handed them back.
I'm still not sure about Rome.
They were ruthless and technically proficient, ambitious people could do well in their structure. The 'haves' had plenty to fight for and even the 'have nots' had something. Food and security. Things well worth fighting for because they make bringing up a family much easier.
Food brought down russian communism, if you can't feed 'em properly, you're history.
Social change occurs at lightning speed.
http://www.acs.ohio-state.edu/history/isthmia/teg/Hist111H/issues/rome1.html
Since not all viewpoints are available on the Internet, this list of theories about the Fall of Rome is limited:
Decay
Financial Problems
The Dole and Barbarians
Economic, Military, Gradual
Christianity
Vandals and Religious Controversy
Division of the Empire
Lead Poisoning
Hoarding and Deficit
My own totally unscientific guess for the fall of Rome is food shortages.(for whatever reasons, drought pestilence, rivalry between different areas etc)
Nothing effects a faster quieter change in social structure than an empty belly.
[This message was edited by eek on April 25, 2004 at 07:14 AM.]