OVM,
Let's see, the empire of the great Khan was the largest empire the world has ever seen, but the price paid for that was the slaughter of millions of people. It fell becuase Genghis died and the empire was handed to his third son who in turn also died and it became a mess form that point on, falling around 1400, only 200 years old.
The roman empire, well we can take serious lessons from them. One of the biggest issue I speak of is citizen involvement, which was one of the factors that led to the split and than later the fall of the empires. The need to put sports before civic responsiblity, the split in religion, the outsourcing of key operations within the empire and the people who became lazy and complacent - all were factors.
The UK? Well they are where we are right now when they started to feel that the great empire was headed down the tubes - this happened some time in the late 19th century with the problems of Indian imports of textiles gutting the English textile industry. Once they got involved with the first world war, they were lost. There is now a serious examination why they got involved and if they didn't, what would have happened - kind of interesting to read that many feel that the world would have been better to let Germany walk into France and Belgium the first time and let them dictate the conditions of peace.
Glad you didn't bring up france.
Oh, if there is a concern about us falling, here is how we really prevent it, stop saying we are a democracy, stop messing with the political system by having elections to elect people, get the people involved with the best political system the world has ever seen and don't vote to make history or fill some gap caused by guilt that one should never have.