Most of you are seriously underestimating U.S. power. The Army teaches that one team (four men) can defeat an entire platoon (40-50 men) if they have chosen the battle wisely and with the proper defenses, terrain, motivation, etc. And everything in the Army is built upon such things that an infantry man can do with his rifle. Make no mistake, nuclear weapons aside, the United States is the only country in the history of the world capable of dominating it.
Hitler's biggest obstacle (outside of his foolhardy invasion of the Soviet Union) was that he had to expand from a small and isolated position in the world. The United States already has its forces spread across the entire globe. Ever see Star Wars Episode III? The Emperor merely makes a call to action and all of his men, who are already in place, make their respective kills and take away the heart of the enemy. The United States could accomplish something very similar.
If we were to form a plan to conquer the entire world, for example, we would move only at the most opportune moment. Personally, I would wait for the Three Gorges Dam to be complete in China and aim enough weapons at it to flood approximately one-third of China. You don't think that would screw the Chinese position in the world pretty handily? We could do similar things in other parts of the world. Everyone talks about 6 billion people versus 300 million. My gut says that, with the element of surprise on our side, we could knock the enemy opposition force down pretty quickly, either by killing them or destroying their countries' positions in the world. Also, in this war, there would be no Geneva Conventions. Instead of occupying countries with our troops in danger, we could do things similar to what the Germans did (concentration camps, martial law, etc.). In short, if we were truly sincere about conquering the world, many of the rules, regulations, and politics that bind us now would no longer be relevant.
The United States has never lost a declared war. If we declared war against the rest of the world, I'll be damned if we wouldn't win that thing. Of course, I don't want any of this to ever take place.