I think that the only thing that can bring the US down is the US. By that I mean that it is in danger of collapsing upon itself. When a hand full of people in power chip away at our gauranteed rights, arrogantly abuse it's power around the world, ignores the will of the masses, when 2% of the population has more wealth then the rest of the population combined, when work hours are longer,the cost of living higher, and wages are stagnet if not declining, when a major US city gets destroyed by a hurricane, is still not rebuilt 2 years later as we spend billions for an uncessary war, etc. etc. Draw your own conclusions. There are countless examples of fallen empires throughout history. The points that I have offered are but a few of the symptoms of teetering empires