Disclaimer: I didn't read all the replys

Politically.... A great Empire's main downfal has always been overstretching its means. There is a point when the empire posesses enough power to be able to control and exploit other nations. Of course, as is human nature, the Empire will use this capability to its full potential, acting at first, as benefactor, then leader, then finally as hegemon. The problem is, the empire will inevitably use its far-reaching potential and piss off the nations it has muddled with. As the empire's power wanes, as it inevitably will, the resentment thats been festering in the subjugated nations will come to a front in the form of rebellion. This isn't to say the subjugated nations are faultless though. They grasp and plead for their chance at the advantage$ of cooperation will bring. They implore hand outs, and in return offer tribute, socially, politically or economically. They let themselves fall into the trap.

There are two types of hegemons. One is the "benefactor," an empire created through diplomacy and maintained through diplomacy (war, only when necessary or particularly profitable). That is what ancient Athens was. That is what the USA is.

The other type is the "pheonix." It usually follows a the slow breakdown of an entrenched system, and through timing and strategy, forms an empire militarily usually within a period of less than five years. The empire lasts only as long as its founding leader, and it soon is followed by a long-term, stable government. Examples of this are Alexander the Great, Hitler (a failed "pheonix"), and the Sui Dynasty in China.

(nice little theory, huh? I just came up with those )

Examples of the phenomena abound (I'm a historian h: ), I can list some, if you like, but youre probably more or less familar with the gist of most of them.

As for the nuclear armageddon stuff. Yeah, its bound to happen. I had this political science/int'l relations proffessor (a real, world-renowned guy) who constructed this theory, based on war data from around the world going back to around the 13th century. Calculating the number of years between what he considers "major wars" (a certain number of casualties), and "minor wars," and what would now be considered "major world conflicts" (world wars, duh), the amount of casualties, the number of factions involved, etc, He created a formula to predict the both the timing and the number of casualties of the next "major war," or what will likely be a "major world conflict." I don't remember the numbers specifically, but its pretty friking terrifying how well presented his argument is.

This phenomenon can be interpreted biologically too. Human civilization has gone through periods of stable growth, speeding up with some sort of revolution (the neolithic revolution, the 3 field system, the double-harvest of rice, the industrial revolution, the etc..) when the population rises sharply (the human population has doubled 1963-2003). This period of rapid expansion is followed by a disaster (the pleague, natural disasters, etc) which either kill off a large portion of the population, or at least bring the gowth rate down to a more stable percentage. Its a well known fact that we are allready way over the earth's capacity to support our numbers. The only difference between now and previous cycles, is the fact that we posess the technology to be our own disaster. Lovely, aint it?