Ask yourself, “Why did the British Empire fall?” The fact is that, it is purely illogical for a country of it’s size to amass such a huge empire. Also after the anti-colonial movement following WWII, Britain’s fate was set. Under these conditions, I would have to assume that the US would have to fragment into many different countries (I have dips on the Communist California), in order to achieve this affect - very unlikely. Even today, Britain has to rely on imports mainly to survive (food stuffs, raw materials). The US has yet to exhaust its wealth in resources (as I mentioned early), and probably never will. That it why I did not put Britain (or Japan) in my equation of stable countries. Their flaw is that they are much too dependent on other countries for even basic necessitates. Also, has the UK completely faded? No! The UK still remains an economic powerhouse, with a high per capita income and GDP. It is unconceivable to think that Britain has clearly seen its days. As for the Roman Empire, that Empire had neither the economic system nor the republican, government system backed up by sound legal codes. Also the Roman Empire was built on conquest, shares none of that it common with the latter (except in the case of the Civil War). There is absolutely no way, short of an act of God that would lead to US’s downfall. That said, certainly US is at its peak and will begin to lose some of its luster (as when the World Bank predicts that China’s GDP will surpass US’s in 2020). However, that won’t mean that the US will remain a very powerful country and certainly among the world’s superpowers.