Cautiously peers from foxhole before speaking. 
Ah, well if I could make a point here....
To address the endgame of sledge's argument: I agree that, if China/Russia/X-Nation were to subdue the majority of the world and use its natural resources for their own benefit, they would eventually be able to take us in anything but a nuclear war. I think it's likely that China/Russia have the capability to overtake Western Europe and the Middle East. I think one of them could overpower the other one, given opportunity/luck. I do think it's unlikely that Russia or China would just allow the other to accumulate vast natural resources via conquest without intervention, even if just to ensure that the other's power does not increase too much to become unmanageable. However, let's assume that one of them has conquered everyone else, or that they have both formed an alliance. They still will not be able to succeed.
The U.S. has the most powerful navy in the world. We have the best military technology (or technology that is the equal of everyone else) in the world. Bringing our money and military back home will only increase its effectiveness via a much stronger economy, meaning strong advances in military hardware and training. The problem with China subduing us has never been a lack of numbers. It's their lack of ability to establish and maintain a foothold on the U.S. to successfully land and supply their several hundred-million troops. Their plan completely relies on a strong naval presence, which neither the Chinese nor the Russians possess. Lacking this presence, they are cannon fodder for our naval, coastal and air forces, only a portion of which would be necessary to successfully destroy all or most of their landing forces. The rest could be used for counter-offensive operations, taking the fight to their home soil. The Russians would be able to land some people here, I would imagine, if they were able to mount a strong enough air contingent to overcome the Alaskan defenses. If they were to land enough troops (or make a window for the Chinese to land troops), they could possibly sever the Alaskan pipeline, severely increasing supply problems for American troops. However, especially considering that the Canadians would also be threatened by this move, I doubt any landed force would last long against a counter-offensive, if they were even good enough to expel our forces from the Alaskan theatre.
All that is a moot point, however, and I'll tell you why. Look back at history - when have you ever seen a nation (particularly a Communist nation) successfully assimilate several other nations into itself and function well enough that it actually increases its efficiency? You don't. The Roman Empire was able to conquer in that manner, but only because the lands it conquered were adjacent to itself or just across the Mediterranean. The more it expanded, the weaker it became. Its supply lines were strained more every day it advanced. It had to commit more of its forces to suppress civil unrest, because just because conquered lands belonged to Rome, the people and the culture weren't "Roman." They were the conquered. This increased the need for replacements. The replacements eventually almost solely came from the "assimilated" peoples, and were essentially mercenaries (not Roman soldiers) who were just as likely to sack Rome herself if they were not kept appeased by gold that the Western Roman emperors had to borrow. Eventually, what was left of the empire imploded upon itself. The modern equivalent might be Soviet Russia. She conquered her neighbors and was checked by the U.S. However, she did not fall because we attacked her. She fell because her own economic and political practices failed. She was not able to sustain an economy that would fund the constant military buildup. Her subjugated peoples were not "Russian" any more than the Germanic tribesmen were "Roman." There was resistance, and eventually the Eastern European peoples regained their independence. The point is this - if China or Russia conquers Europe and the Middle East, all the natural resources won't guarantee success against the U.S. It would take a phenomenal amount of resources, manpower and coordination to take us down without nuclear attacks. The resources and manpower they would have, but the coordination they would not have. To think that the European and Middle Eastern nations would blindly submit, instead of reawakening a fire of independence and starting resistances and revolutions, is just silly. Human history proves that empires of this magnitude cannot long last and maintain their effectiveness. As expansion occurs, the effectiveness diminishes. They will not be able to take us if they conquer everyone else, sledge, because if they do they'll be doing everything in their power just to keep their empire functioning at its basic levels.
A great example of current overextension is us. You say that we're stronger when we're projecting by interventionism? I believe you're wrong. As EJR has already stated, we're spread too thin. We're more vulnerable now than we should be, because we have our fingers in too many pies. Our military is committed across the globe. The British Empire once was as we are now...they had a small but effective military that was technologically superior to their enemies. However, they were kicked out of many other nations and are now reduced to that small island. If we don't keep our military organized and strong, we can be defeated piecemeal. It's only a matter of time.
So, to recap...China and Russia could not hope to conquer all of the world and then come for us, because they'd be too occupied keeping their empires functioning. We cannot project our strength like this much longer without someone taking advantage of it and hurting us, making us bleed. The only solution seems to be to recall our troops, defend our immediate sphere of influence (the Americas), and become strong once more.
@EJR: What exactly did we disagree about? I thought we see eye-to-eye on pretty much everything...did I forget?
