Interesting questions. The history I read suggested Lead Poisoning as the cause of Rome's fall, not the Celts who came back after 700 years and finished what Brennus started.

The USA already well on the way towards internal collapse - see Israeli influence in foreign policy, often utterly against the long term interests of the average American - blindly follows a policy of so called liberalisation of certain regions of the globe; oddly these very regions don't want to be 'liberalised' and instead cling to a fundamentalism far far more potent than any Babble-on kind we already know in the west.

Perhaps this is the real challenge which the USA must first confront then later be froced to accept defeat.

There remains yet one factor ignored - wondering why? - by most discussions, the environment. Already we are living on a planet well on the way to radical change. Our greedy automobiles/engines eat away at the very stuff we need to survive, the air, by the cubic MILE and nobody seems to connect weather, health eg SARS, and other phenomena with it; morover those who do connect these things are fast shut down, or up, by Corporate Amurika which shouts down with Massmedia bullying any attempt to question our plight.

Perhaps that alone will first weaken then destroy the most part of the Americas leaving but Mountain regions as habitable?. In any case the arrival of Corporate Industrial mismanagement to China, esp Ag products, together with the vast numbers of carbon fuel gobbling engines sold, explains to some, not blinded to truth, what is about to happen. Put it this way, Hiroshima was irresponsible because it excused the environmentaly inexcusable for politics and HUMAN GAIN,
Mal Wart USA will SELL YOU FRESH AIR at the same time it finances the means to destroy it - the Chinese pollution of Earth's mantle. It is greed and gain for it's own sake, let those who cannot afford to move to the dwindling islands of clean air die where the choke.
Mal Wart USA... oh yeah!

Los Angeles, New York, Seattle, Miami..

In that scenario Water does not matter, however it would if we survive being choked by Chinese wind driven clouds of Oxygenless Nitrogen.

Another possibility is being discovered by some Alien race which may or may not eat us, consume our planet, perhaps they may arrive take one look and leave. In anycase the possibility is so remote as to be irrelevant.

I vote for a Tornado erasing the White House off the face of the earth the very day that the Republicans decide to go ahead with Program China Air (lessness)